In theory you don't need any paperwork or training to use your boat around the coast of the UK. In reality, it's a good idea to have insurance, some basic training (ideally an RYA powerboat level 2 if you're a beginner) and a VHF radio licence (a legal requirement if you plan to use a VHF radio on board, which is a good thing to have in case you get into trouble).
In terms of launching, different slipways have different rules and regulations. For example, if you launch at the Camber dock slipway in Old Portsmouth, you have to call up the harbour master on the radio to get permission to cross the harbour and head out to sea. Other places have no regulations at all. Your best bet is to buy an almanac which will have details of local regulations, and have a look at boatlaunch.co.uk which also has lots of info.