If your flight's on time, 41 minutes is enough. Depending on how late it is, all bets are off. I always try to have a longer layover due to weather, but obviously that's not always possible. I do agree w/ the others though, at that time of the year you should be ok weatherwise. And yup, SFO is better than LHR for this type of thing. Have fun!

Cheers.

P.S. Bad news - you won't get any champagne on those flights.