Hi Kizzie.
Firstly, you can pop "heathrow tour" into the search form
here to get a look at some recent threads on the topic - there have been some really useful contributions made, iirc, including something of a debate on the nature of a Heathrow tour (busy vs. dead etc) and I think a mention of the fact that if it's dead, and you're stuck out by a tedious airport, you might not have much to do, so watch out for that! But it seems likely that with proper advertising and general setup, you won't have too much free time on your hands.
As for London, I live here and I think business is excellent, but that comes with the usual caveat that I do 5 appts. a week max. I usually have interest for more bookings, though, and have been booked 5-7 days in advance for some weeks now. I think London is a flexible and large enough market that anyone can find their niche and make a comfortable living here (if I can do it, anyone can, literally!) but I'm obviously not touring so that might all be rather moot. I do know certain women of this board do tour London extremely successfully so again, probably a search for "London tour" would find you that information!