Your basic ppl will allow you to fly single engine aircraft (C172, Pa28, etc) You obviously need to be checked out on each different aircraft you fly. In order to fly twins, you need to get a Multi rating. Usually the next step is to get a Night rating followed by an IMC rating to allow you to fly in cloud. These cost around £600 & £1800 respectively, depending on where you go.
If you are planning on flying commercially, there are several different routes, but it is very expensive if you are funding it yourself. Most people will do the required hour building and their ATPL (Airline Transport Pilot Licence) ground exams followed by a CPL course. The next stage might be to get an Instrument Rating (IR). This is probably the most challenging part of flight training.
I suggest you buy a copy of LASORS from the CAA. It'll probably be the best £10 you've ever spent!
