If CS is important, you should go with a good package from an independent such as Zen. Most good packages are wires-only, which is a good thing in my opinion: I would rather choose my own hardware. £15 p/m is possible, but limits your choice - the best providers cost a little bit more than that, but you get a lot of features (better than free phone calls), good service, and often better speed with little or no throttling.
Yes, you will say goodbye to current e-mail addresses; that's one reason why quite a lot of people use addresses on their own domains or free webmail accounts. I opt for the former.
You not a light user, particularly, but you really should keep an eye on your bandwidth usage so that you know what you need. I have 20GB p/m and fill most of it with uploads - you won't, so I think maybe 5GB will suit you just fine. But please check.