I've bought from David Stuart in the past and had a good experience. I'm across the pond, so I imagine it would be even more painless for you. His website is: http://www.abccoinsandtokens.com - he sells a bit of everything, but specializes in coins from the UK and it's various divisions, new and old.
I live in England as well. If Celtic is your thing there is Chris Rudd. Also if you Google coin shows a list usually comes up.