All I can say is that it is lucky that English doesn’t have mutations,
I’ve recently attended an excellent course of talks by @HarrietEaris in which she convinced us that English does indeed have mutations. Unfortunately I can’t remember her examples. Perhaps by tagging her, I may prompt her to enlighten us.
There are lots. For instance, according to the dictionary, the ‘e’ in ‘the’ is supposed to get longer before a vowel, so the ‘the’ in ‘the end’ is longer than the ‘the’ in ‘the camel’.
Thanks, John. I’ve just asked Harriet for further examples.
It will be great to see them Hewrop.
I can’t think of any gramatical mutations at the moment, but softenings happen without even thinking of them, e.g, roof -> rooves, North -> Northern, etc.
That’s before you get onto inconsistent pronunciation. ‘Bow’ in ribbon, ‘bow’ as a sign of respect, ‘beau’ as in love interest, ‘bow’ as in weapon, ‘bough’ as in branch but sounds like sign of respect. Then ‘beau’ gets put into ‘beautiful’ and totally changes. In ‘house’ the ‘s’ sounds different to the ‘s’ in ‘houses’. I suppose you have ‘wife’ becoming ‘wives’, too.
There was a ‘thing’ that I think goes back to Oscar Wilde… how should you pronounce ‘ghoti’? Well you take the ‘gh’ from ‘tough’, the ‘o’ from ‘women’ and the ‘ti’ from caution and come to the conclusion that ‘ghoti’ sounds like ‘fish’. Mad!
I’m not sure that you really do get mutations in English, although ‘rooves’ might be the best example of the nearest thing. You do get assimilation of various sorts, as you do in most languages - where two sounds coming together cause one of them to change to be a bit less unlike the other. Quite often it’s the first sound that changes, as our mouths and brains get ready for what’s coming next: the ‘m’ of ‘comfort’ is actually made with the bottom lip and top teeth, like the following ‘f’, rather than with the two lips, like the ‘m’ of ‘mother’. (If you try to do a labiodental ‘m’ out of context you’ll just look like you’re doing a chipmunk impression.) Sometimes, though, it’s the second sound that changes - like the ‘s’ sounds of ‘cats’ and ‘dogs’, and it’s that kind of assimilation that really underlies Welsh mutations.
But then what happens is that languages change, and so the condition that leads to the assimilation can change or disappear. In Gothic the root form of the word that corresponds to Modern English ‘loaf’ was ‘hláib-’ (‘hl’ similar, but not identical to, Welsh ‘ll’, ‘ái’ like the word ‘eye’, ‘b’ like Spanish): it, or something like it, got borrowed into an early Slavonic language to give the ‘Polski chleb’ signs that you see outside Polish shops. When this ‘b’ was at the end of a word, or followed directly by an ‘s’, it became ‘f’ (Gothic hláifs, Old English hláf, Modern loaf); where it wound up between vowels, it stayed as a Spanish b/v (Gothic hláibos) or turned into a ‘v’ sound (spelt ‘f’ in Old English hláfas). In English this ‘f/v’ pattern has stuck, even though the following vowel has become silent (Modern loaves); and because it was quite a common pattern (thieves, leaves) it’s even got extended as a grammatical pattern to other words where the dictionary says we “shouldn’t”, such as ‘rooves’ (standard ‘roofs’).
So what happens in the Celtic languages basically follows this pattern, only much, much more so. A word like “yn” (=‘in’) obviously ends in a nasal sound, so the mutation in a phrase like ‘yng Nghymru’ isn’t really anything more than plain old assimilation. But then, presumably, a word like ‘fy’ must once upon a time have ended in a nasal, otherwise ‘fy ngardd’ and ‘fy nhad’ make no sense - so now we’re doing an assimilation with a sound that’s no longer there! In the same way ‘ei’ (=‘her’) and ‘ei’ (=‘his’) must have once ended in different sounds, or else it’d be hard to account for ‘ei phen’ and ‘ei ben’. But now we’ve got a situation where these changes, made in response to sounds that vanished from the language about the time of King Arthur, can in and of themselves convey meaning; and that then leads us on to changes like ‘chlywais i ddim’ (‘I didn’t hear’, from ‘clywais’ ‘I heard’), where the word ‘nid’ (‘not’) - which has vanished - used to have some sound on the end - which has also vanished - that caused a following ‘c’ to turn into a ‘ch’. That change in its own is now fixed and meaningful, such that before you even get as far as ‘ddim’ we know that you haven’t heard, even though all the original, underlying, historical reasons for that change basically never made it back from Catraeth 
So, yeah… we can sort of find phenomena in English (and other languages) that kind of parallel mutations, and ‘rooves’ is probably the best example but, frankly, it ain’t a patch on Welsh
To be honest, plurals like rooves and variations in pronunciation are not what I recollect from Harriet’s talks when it comes to mutations as they exist in Welsh and Gaelic, for example. I’m seeing Harriet at her harp concert in Llanilar on Sunday and I’ll ask her to refresh my memory (which is becoming more and more unreliable).
Tech question. Can someone tell me how to split this mutation side topic (mea culpa) to its own thread?
Edit: I seem to have the appropriate trust level and I have managed to work it out myself. I hope this is acceptable to all valued posters. If not, please PM me. 
An S sound very often changes to a Z sound at the end of a word. CatS and dogZ. Margaret’S clotheZ.HiZ bikeS.
In southern Welsh, the fy commonly becomes “yn” - you’ll hear it quite a bit in the southern courses, where Cat carefully says “fy”, and I say “yn”. I suspect you are right that fy used to be fyn, although I’ve never seen the theory proposed by anyone who actually knows anything!
Ah so when T. Llew Jones writes “… 'y nhad i…” is it said “yn had”? I’ve been wondering about this, I just ignored the y and said “nhad” in my head.
This is Harriet’s reply to me. It appears that @johndray and @RichardBuck were much closer to the mark than my wobbly memory.
The things that come to mind where English has mutated consonants to ease pronunciation are:
(a) “walked” or “talked” where the “d” is pronounced “t” (d>t).
(b) internal mutations like “maternal” but “mother” (t>th).
(c) American English “water”, “butter”, “metal” is pronounced “wahder” “buhder”, “mehdal” (t>d).
(d) Cockney “wha’s the ma’er?” for “what’s the matter” (t>th>h>glottal stop).
(e) “light” but German “licht” (c>ch>gh>h), “night” but German “nacht” (ch>gh>h)
I’m sure there are hundreds more. It’s the same principal at work, it’s just that English tends to do it with letters at the end or middle of a word and Welsh does it with letters at the start of words…
Apart from being a Celtic Languages graduate, Harriet is a virtuoso harpist. She and Nick Wiseman-Ellis, a button accordionist, are giving a concert tomorrow (Sunday 2/7/17), 3-5pm in Llanilar Village Hall, Ceredigion. (£10/£8 at the door) Say hello, please, of you decide to come - see my avatar.
Interesting – I hadn’t come across this (I’ve been resolutely Gog on SSiW), but I’ve just looked in my Middle Welsh Grammar, and it quotes forms (in the spelling of the day) vy, vyn, and y, with plenty of examples, including things like uy gwlat, vyg gwlat, yg wlat all for fy ngwlad. So I guess (a) that your variant is at least hundreds of years old, and (b) that explains @AnthonyCusack’s 'y nhad i for fy nhad i.
That’s an excellent example.
Also - forgot to say - shows I was right about fy ending in a nasal, but entirely wrong about it having gone the way of King Arthur. Unless, of course, King Arthur is alive and well and living in Llandysul
The glottal stop is also common in the NE of England, although You wouldn’t dare call it that. Although I’ve toned down now, I still tend to say “compu-er” and “calcula-er” if I don’t watch myself.
There are cases of politicians deliberately adopting the glottal stop to gain “street cred”. The one I have in mind sounded really weird.
You have reminded me of the Spitting Image Nigel Kennedy song:
“I talk like this ‘cos I fink it’s righ’ on
I’m the world’s first Cockney born in Brigh’on.”
Ha ha. Mind, you, it is easy to loose/pick up an accent, even in songs. It’s difficult not to sing a song in the original singers accent, be it from an American, Australian or British region. Come to that, I’m sure that I’m developing a fake Gog accent now. 
When I was in Newcastle upon Tyne, there seems to whole different words. So instead of ‘he told me’ (S) you would get ‘he telt wah’ (N). I’m not an expert but I wonder if they both came from something like ‘he telled me’.
“Myn uffarn!” is frequently uttered by Capten Hadog. “Cocls Ceinewydd!” is probably a bit politer.