Hi Tim
From a sunny southern California perspective, (however it is pouring rain at the moment ), London weather is always cold and wet.
Personally I like visiting London in March/April, attractions aren't too crowded and the hotels are still offering off-season rates, something to consider with the dollar's decline; mid-week flights are the better way to go. As far as a route, I've always flown out of LAX (always made it, always got first ) however with my D3's I usually go with the odds and route them through JFK, six chances are better than two.
You might want to drop a line to Paul the United Kingdom correspondent for info on events and holidays.
I'm off to find an umbrella!

Keep'em flying