I sweat like a pig too, and I'm not that heavy. Of course, it's about 38 degrees celsius right now... so, that could be a factor. Also, as the summer goes by, if you exercise consistently, your sweat gets less salty and the quantity increases. It's an adaptation to keep you from losing minerals while keeping you cool. I sweat about a gallon of pure water whenever I play basketball or ultimate.
Sweats healthy, don't worry, unless you're sitting still in a cool room.
Also, are there many good places to play basketball in London? I'm going to UCl and this has concerned me. Pm me if you want to keep this thread on topic.