Well, London has plenty of WG's to choose from, so click on the regional forums and start reading reviews. Find a review that you like the sound of, and then copy & paste the name of the WG into the UK Punting search box to see what other mentions there are of her.
Remember, what one man considers to be his perfect WG may not be to your tastes, so don't assume a glowing review will represent exactly what you are going to get.
Your first time will very likely be an uncomfortable and awkward experience, so don't feel disappointed if the experience goes a bit pear-shaped. I suggest visiting three WG's - maybe one a week. Physically losing your virginity is easy, but emotionally it doesn't happen all at once, it's more like a learning curve.
When you search the AdultWorks profiles, be savvy about it; many profiles are fake, many reviews are fake, so you have to use your head. If it looks too good to be true it usually is. Stick with the London area, there's no point traveling very far and getting stressed out and tired before you even get there.