Well I've been to London, but never to gay clubs.
Most clubs are located at Leicester Square. There are high end clubs, but there are also just bars there. Club Metra, Oxygen were all good.
There was a club at Leicester Square at the Penthouse. This one was really good, though the tickets aren't on everybody's budget, like £50.00. The music was really good though.
There was this one club Funky Buddha
which I found on the internet had really great parties, but I wasn't allowed, because I wasn't 21 yet at the time. The door policy is real strict and they check everyone's ID. Some parties have strict dress codes.
One time I went to a club called AKA The End, but I didn't know it was a drugs club. Everybody there was on to something. I think it was an XTC party, so I would stay away from there, if that is not your deal. You should also keep in mind that to most clubs you can get free entry. The promoters always approached my friends and I and gave us free tickets. Meeting up with girls and getting in together will upgrade your chances of entry. I think I only paid for entry 1-2 times, but got a free drink inside.
You should be aware though that people will come up to you and ask you if you fancy a Charlie, or a Peter. Those are code for drugs.
There are clubs at in the Soho neighbourhood worth checking out as well.
If you want to save money on inner city travel, you can buy an Oyster card and travel with the metro or characteristic red double-decker busses (I recommend).
I hope you have fun and enjoy your trip. Sorry I don't have information about Paris for you. I have been there only once and it was really short, so can't give you proper information. Maybe someone else can.
I found this website for you http://www.yell.com/reviews/london-leicester+squarQ-club-night+cls-