If you have Sky TV already then the cheapest solution would be Sky Broadband - if in an available area this can be free with speeds of upto 2 meg - this may be sufficient for what you intend to use.

O2 have a very good record in terms of customer service and reliability - this would be my recommendation if you are looking for service over cost.

From your intended usage I would think that you need speeds upto 8 meg for good quality online gaming, less if you dont intend to use this service much

Any futher questions please let me know

thanks