There are lots of london agencies and it really is easy as calling them up and making a booking.
I have used Admiral Escorts which you have quoted and there is another one called "Hot Collection".
If you are staying in London, why not go for an incall? Cheaper and with the tube London is easy to get round. Use googlemaps or an offline gps map to locate the address. The way it works which you probably know is you call them up, tell them who you want to see and they text you the address. Most of the places are plush apartments and you just ring the buzzer with the flat number and waltz in when they buzz you in .... dead easy
I don't know your preference but here are a few I have seen which I think are good:http://www.hot-collection.co.uk/escort/charlottehttp://www.hot-collection.co.uk/escort/zuzahttp://www.hot-collection.co.uk/escort/lurbnci8r7http://www.hot-collection.co.uk/escort/caroline45
I've quoted the Hot Collection site but check out admirals also; there are loads of agency websites and girls do appear on different sites. Worth checking just to make sure you don't pay more than you have to; ie, different agencies may have different prices for the same girl.
There is a large selection, it is often overwhelming.
Based on my experience, the above girls provided a great service.
Good luck! Hope that was helpful