Living in Surrey, I would have thought you would have hard water with a high PH (trickling through all that chalk!). Livebearers should be fine with this and are also easy-going (and not expensive) fish. People have already suggested swordtails and platies- I would go along with either of those (used to keep them years ago, and have just bought myself some platies, my tank being too small for swordtails, yours should be fine). Platies always seem very pleasant fish to me, inquisitive but not aggressive, and without colour variation to tell them apart. Swordtails are nice to, add a bit more action to a tank.
Danios (again not expensive) are another good idea, you would have room for a shoal in there, which would look good, though you would have to check how they are with your PH.
The hardier corydoras (as suggested in other posting) should also be OK, bronze or peppers. They are great fun to watch if kept in small schools.
Neon tetras are sold very cheap, but they are more sensitive (as I know to my cost
![Frown :( :(](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
) and they don't like hard water or new tanks, so you're probably better off without them (or rather, they're better off...).