Re plasterboard boxes, if you are going to use them, use Appleby. They are the only make I trust not to disintegrate or jam. You can get metal plasterboard back boxes, TLC sell them, but they can be a bit of a PITA to fit.
You are not supposed to fit "general purpose" sockets less than 300mm from a corner. Note that word "general purpose" My take is , providing you have enough "general purpose" sockets to meet the regs, and you have a need for one right in a corner, then fit it. If your building inspector is a jobsworth and complains, put a label on it "for tv only" and it's no longer "general purpose" This is the same rule that lets you put sockets high up for a wall mounted tv, as it's not "general purpose" You only need something like 4 "general purpose" sockets in most rooms so it's easy to get enough that do meet the accessibilty rules.