My answers come courtesy of Richard Cutcliffe with RhinoCo security specialists (call 0845 6445421 if you have more queries) and the Information Commissioner's Office (ICO) and its website (although that website ain't for the fainthearted it took me at least half a morning).
Answers: There is no requirement for an installer to be regulated. It's probably best to go for a local supplier so if things go wrong you can get your hands on him. And if it goes wrong again, report it to Trading Standards they should be siding with you. If your supplier says you need a service agreement to comply with the DPA he is being creative about your requirements. The ICO says you don't have to register if you have only one camera for security purposes, but then directed me to their turgid website for confirmation. The best advice is to call direct on 01625 545745 with specific queries.
On the subject of Redcare, it is likely to remain the standard for some time. Your insurance company will probably be the first to inform you of any changes.