a. Start looking now. It doesn't hurt, and if you find an SFO-LHR deal that sounds amazing for that time of year (say, under $400) jump on it. I'm looking at going to the UK around the same time, and right now I'm finding $500-600 to be the norm. There's a good chance it'll fall below that at some point, though, but maybe not for very long.
b. I love Virgin and BA. BA has better legroom and food, but Virgin has the best in-flight entertainment and a frequent flyer system that earns rewards much more quickly. Not to mention the hottest flight attendants
Aaaaanyway, sign up for specials at both their web sites. You'll find that when one of them has a sale, the other will try to match or outdo them. I got from SFO to London for $200 that way a couple of Novembers ago.
c. Hell, sign up from emails from other places, too - I like travelzoo.com's weekly alerts in particular. I've had a few spur-of-the-moment trips because of those. *sigh*
d. You're probably talking about EasyJet, and they're nice for a no-frills airline... and they fly out of Heathrow, which is a big bonus compared to RyanAir. Just remember that luggage weight restrictions are tighter on budget airlines than on the major carrier you take to London, so pack accordingly.