Add a box to the right of the existing one, your cut-off cable will then be long enough to enter that one. Then add a double pole fused spur, to the new box, fed from your working socket. That new spur, has two advantages...
1. If your outside socket developed an issue, water ingress etc., and began tripping the circuit, you have an easy method of isolating.
2. You can turn it off when not using it, to prevent anyone stealing your power.
3. (did I say two?) it resolves your cut short cable problem.