I've been researching this (I'm a Londoner too). There are several hair places in London but they mainly like to work with their own pieces- that's the rub. However, I have found a couple that will cut a Toplace piece for you:

1). Hair Development in Mile End, East London (they charge £50).

2). The London Hair Clinic will also work with a piece that isn't their own but be warned their prices are steep. £130 the last time I checked. http://www.londonhairclinic.co.uk/

I haven't been to either, I went to a place that did a one-off, so to speak, for me.

RE: Forever Hair- I didn't like the cut-in I had there. The stylist used hair-straighteners on my piece! The piece looked terrible. But people seem to recommend it.

Please let me know if you come across any other hair piece stylists.