I have asked this thread be moved to London.
Admiral is the most visited agency site in London. (Of the 95 or so listed on the Punting Wiki). May or may not be the best. Is McDonalds the best restaurant? But not saying it is like McD, just saying popularity does not mean best.
If you look on the London pages, there are lists of girls by nationality that have multiple positive reviews here on this site. Most are less expensive than the one you mentioned. You might want to look at some of those too - perhaps mention a few others you like and ask any specific questions not answered in reviews.