Not too sure about the stage version, but in the film version there's no nudity or sex, just the "orgasm" scene. I can't imagine that Alyson Hannigan would do any nude scenes either. Plus the plot of the show would make any nudity/sex really gratuituous, seeing as the main couple are supposed to be platonic friends for most of it.
Hope you have a good time in London! (PS. If you check broadway.com sooner to the opening, WHMS should be listed under London Shows and it'll say what ages it's appropriate for.)
"It's not always about you!!!" (But if you think I'm referring to you anyway, then I probably am.)
"Good luck returning my ass!" - Wilhemina Slater
"This is my breakfast, lunch and f***ing dinner right here. I'm not even f***in' joking." - Colin Farrell