You can enjoy sightseeing a lot more during the summer. Open roof bus tours, river cruises, and walks are great on a sunny day. You can also get a better view from the London Eye on clear days, whereas in the winter, the rain or grey skies might make it harder to see. I would recommend July or August as one of the best times to visit.
I visited London last winter and was amazed that it wasn't nearly as cold as I expected it would be, although there was a bit of rain. I'm sure summer must be lovely there, but based on my visit, visiting in winter really wasn't a problem either. Actually, it was a welcome relief from the 40 + degree temperatures back home in Perth, Australia.