The same rules will apply at Heathrow-that been said I travel frequently between the UK and US and have never been patted down in a way that would "expose" a wig (Of course Murphy's law will come into effect now so watch out for my post next week -TSA wanted to search my head
) Seriously please don't let your fears spoil your trip you have NOTHING to worry about. I do understand about psoriasis my Mum suffered from it too.
I hope you'll post to this thread when you return from holiday so anyone else with similar fears can benefit from your experience.