This is hard to answer many of us would cheat. The problem is although you could use crimps to use them in such a limited space is very hard. And it would need proper crimp pliers not the cheap DIY type. Choc block would present the same sorts of problems due to limited rooms. So soldering and putting shrink sleeve over the joint (type with resin in so it can't slide after fitting) is about the only method that can be used in such a confined space. Even then it takes the flexibility out of the wire. And we are talking about skill and I am sure I could do it, but as to telling someone else to do it I am not sure. I would consider moving the socket box to match cables as even with soldering if in the future some one opens the box being that much stiffer it is easy to either damage the joint or pull cable out of socket.
So moving socket box would be best and only if it can't be moved due to other items on the wall would I consider extending cables. If I did extend cables I would normally use a deeper than normal box.
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