As we have broken up for Christmas, I hope it’s ok go slightly off-topic with this. Two words in common use in the NE of England are:
“Wall”. Pronounced the Welsh way: “Wal”. There’s even a town with this name.
“Lonnen”. A very rural lane, often tree-lined, or a modern road which follows the route of an old lonnen.
OK, perhaps these are more Brithonic, than strictly Welsh, but it is Christmas. Any more you can think of?
Corgi… 
Isn’t “cwm” known in mountaineering circles? (also cf. “combe”).
Also “crag” is Celtic, presumably related to “craig”.
Edit: And of course, the river Avon.
Some people may know “crwth”, a musical instrument.
And “bard” and “druid”? Though those did not come into English from Welsh as such, but from other Celtic languages (“Old Celtic” via Scottish for “bard”, Gaulish via Latin for “druid”, according to EtymOnline).
Penguin.
I really hope that isn’t a myth - Welsh sailors seeing them first but mis-remembering about which bits were black or white!
Well I guess that cariad is a pretty well known word throughout the british isles as well as eisteddfod which are words in modern welsh that are known in english, but I’m sure that there must be many examples from the mists of time where the modern english word derived from a brittonic root.
On a slightly different note I think that I’ve managed to get almost all my children (5 out of 6 ain’t bad) to use Sion Corn this christmas rather than anything else!
Somebody told me the other day that there are types of penguin which do indeed have pennau gwyn, and suggested they might have been the first ones named… which I like enough to choose to believe…
According to Susie Dent in countdown a few years ago only two words have made it into English from Welsh, both of which have been mentioned here already … penguin and corgi.
Coble. A type of fishing boat. From ceubal.
But i just wish pasti was one
That’s fascinating, @Pete2 . Giving me a spot of hiraeth there, as the coble was a very common site where I was brought up. It would certainly be pronounced “kerbl/ceubal” in the Northeast of England.
Anyway, back to Wales: The only present reference to ceubal that I can find on the net 
is Gabalfa in Cardiff: Ceubalfa - the place of the boat, which would have been a ceubal ferry across the Taff at this point. However, it seems that coracles are used all over the world.
Cromlech is one…and crag (from craig) another (and what is flummery again? )
People get tooooo easily confused by Penguin/pengwyn…because they are unaware the original penguin was a common name for the Great auk…a now sadly extinct large bird of the coasts of North West Europe…fisherman simply saw the Antarctic penguin birds and confused them for the Great Auks…hence why Penguin has ancient roots