Nothing illegal about sex in a plane, provided that there is a pilot at the controls.
However, I think if somebody started a business offering to take people up for this purpose and obviously charge them for it, it would attract the attention of the CAA.
AIUI, anything commercial beyond "trial lessons" needs an AOC, even if it is A to A and not A to B.
The way "trial lessons" stay legal is by allowing the passenger to take the controls at some stage, but in this case I don't think that could be used because the majority of people, especially British females, would not be able to transfer between the front and the back in anything smaller than a King Air.