If you really want to take legal action, then the normal course is to first send a "Letter Before Action" (Google it for examples). Then if you don't get a satisfactory response, it's up to you to go to your local court to issue a summons, under the small claims procedure.
You need to work out what claim you need to make - just not liking the service isn't going to get you anywhere.
You can get a lawyer if you like, but don't assume that you will get legal costs repaid if its a "small claim".
But it's certainly worth trying to work through BT's complaints process first before getting into the legal stuff.