Indoor cycling in a group environment is a great way to get fit and to stay in shape. You get all of the fitness benefits and cardio exercise that you would on a normal outdoor cycle, but without getting wet, cold and without the danger of traffic.
The benefits of attending group cycling classes are: 1) You get an instructor to make sure that you're exercising safely 2) You exercise with like minded people which helps to keep you motivated 3) It's a good way to make new friends
There are some excellent group cycling classes up and down the UK.