Try Photogirls, maxesangels or UKsugarbabes!
Alexis Silver is my personal favorite, she advertises on adultwork

I think Rita Faltoyano is usually available in London.

Robin Truelove lives there.

Both were available through Photogirls last time I checked and I think both have reviews here if you want to check them out.

Re: Visiting London

If she's in London when you are, number one should be Tanya Tate, who occasionally posts here. US star and Tanya's good friend Jordan Kingsley may also be here  at the same time.

Harmony Hex is often in London, and currently seems to be running a few specials. Not seen her  yet, but I intend to.

A bit further out of town, in Loughborough, but highly recommended is Tamara Grace.

Other porn stars in the capital or nearby include Cyprus Isles, Kerry Louise, Jools Brooke, Karlie Simon, Suzie Best.

All of the above girls are available independently, some through agencies  too. Contact details can be found on Adultwork, or try searching out a post I did a year or so ago. You can also see pics of all on Killergram, most on Television X's site too, or freeones.