You need CAP413
(1 - edited) You don't have to request it, but you should advise them first if you are receiving a RIS. However, it's a good idea to tell them what you are doing even if you dont have to and I would always tell them if given a specific routing (e.g. not overhead Odiham) because there may well be something else going on.
(2) It's mostly academic, because you'll almost always get told "negative, flight information service only due to workload" when you ask, but with a RIS you must advise the controller before changing level or route. If you have a FIS you don't have to.
(3) tell them
Seriously, if you're unsure about this stuff read Chapter 7 of CAP413 - it explains it all very clearly