Getting reviews on London girls is hard, and my experience with agencies has been poor. There is another thread about how lucky we are to have PERB, and it's accurate.
I’d stay away from the lower tier like Strawberry personally, that seems way too cheap. Read the descriptions of the girls and avoid “party girl” references, unless you’re into that world. I was shocked at how many agency girls have it on the menu. Even the girl I ended up seeing, who said she wasn’t, clearly was if you can read between the lines of my comments. There are a couple of well known agencies that have “reviews" on their websites but take it with a grain of salt.
One thing I did not do that I should have is this. While you’re in London go in to a big hotel, look like you’re a guest, slip the the concierge a few pounds and ask. They see the ladies and likely know a few.
Adult Work has some reviews of indys and can be a good place to start, but the problem I found in looking at indys is that I had no way of sending even a small a deposit. If you have any high end regular girls you see here you might ask if they know anyone who has moved to London and perhaps seek a reference.
I wish I could be more help, it’s very frustrating to have the time alone in a city like that and the desire for someone new to see.