You cannot connect a socket to a light switch mainly for two reasons;
1. The cable supplying a lighting circuit is not large enough to supply a socket outlet. This would be dangerous and against regulations.
2. Behind a light switch there should only be a live wire, a switch wire (live when switch is on) and an earth but no neutral. Therefore your socket won't work anyway because of no neutral.
You need to supply the new socket from the ring circuit, preferably the same one supplying the other sockets in the bedroom, so that when somebody isolates that circuit ALL sockets are isolated. This will probably involve lifting floorboards and breaking into your ring circuit. It is best if you can extend the ring (i.e keep the new socket as part of the ring). If this is not possible then "spur" off the ring by cutting one of the ring cables, rejoining it in a JB (junction box - 30amp) and connecting a (single) new (2.5mm squared) cable into the JB and routing it to the new socket and connceting there.
Always make sure any circuits you are working on / cutting into / connecting to are isolated before starting. The circuits must be checked afterwards for ring continuity and especially that the earth is connected correctly and has low enough resistance path to operate the protective devices at your distribution board.
From the question you have asked, I would guess that you do not have the necessary skills for what can be a dangerous job and one which must be done correctly so as not to endanger people using the socket. My advice would be to get a qualified electrician to do the work instead.
Steve, June 2010
