Where in London could I take my wife where .....

....she could be offered oral sex by another woman, in a gentle environment for shy people?

I think in the right environment she might go with the flow......

I guess you could get lucky by placing an ad, or maybe she would enjoy the idea of another lady giving a tantric massage, perhaps leading to more. It is always slightly more difficult when your wife is supposed to enjoy something that is essentially your fantasy and wish and it may not be hers at all.