For a first time, I'd definitely recommend a Thai. I've seen a few now. I just scanned London on BP and the following recommendations are ones I've seen and had a great time with. One Brazilian thrown in as the exception.
External Link/Members OnlyMillie/Junius. I saw her in Boston and Lincoln for £80 (prices now higher). She's very submissive and feminine. Cock size 4 inches. I paid her a tip after making her jizz.
External Link/Members OnlyIsabella. Changes names often. Extremely hot and feminine, rushes you a bit but very energetic. Cock size 6-7 inches. She's a bit pricier but I saw her twice and she came in about 2 minutes of riding me, making me wonder if it was a bad mistake to pay the extra 40 for her cum.
External Link/Members OnlyIf you want to bridge by just having a massage first, this one should be a safe bet. I saw her before she stopped doing full service and got her to cum hands free. But she only likes to do massage I think.
External Link/Members OnlyVery gentle and soft Brazilian, versatile, willing to please, lovely aura. Cock size 6 inches. She did everything, DFK, OWO, came for me, all in the 100 quid price.