How to add a plug socket in a specific scenario
Although I'm very curious as to exactly what is going on here, in answer to your real question:


This is probably on a lighting (or similar) circuit, which would make it unsuitable for a plug anyway. You could test this by turning off your breakers one by one and figuring out what's on that circuit. If the wiring is up to scratch, all wire under that circuit should be sized appropriately regardless of its actual use. 
However, even if it is suitable for a 13 amp plug, you won't have enough space for all those wires, a block connector and a plug unless it's insanely deep behind there
Blanking plates aren't generally put in "just in case" - it's just as easy to cut the wire and put a socket in, the hard work is getting wires to it and the backbox in. They're normally either left over when something is removed (Such as a hard wired appliance) or, in this case, when the electrician needed somewhere accessible fro a junction box. It was not envisaged as a place for a socket.
Given the above, the complexity of what you're looking at and the fact you're unsure (And I'm just some guy on the Internet guessing) you MUST get in touch with an electrician to investigate this further. You're in this because you were under the impression that it was an "obvious" place to add a plug - I'm telling you that this is probably not the case.