I'll tell you, though you need to be a competent person, and not a job for the DIY man.
Find socket on an outside wall that is covered by a RCD, turn electric off to that socket. Remove socket front, Take knockout from the back of the backbox, drill hole in wall to outside. Put grommet in knockout of box, push 2.5mm T&E cable through hole. Seal hole through wall up with silicone.
Terminate outside socket, ensure sealed good.
Terminate inside socket (all red & brown together in L, all blacks and blue together in the N, put earth sleeving on bare earth and put into the earth) and test...
Also, 2.5mm T&E can be bought from wilkinsons by the mtr, think its 50p a metre.
Wickes sell the grommets in packs of 16 or so, and can pick up earth sleeving for about a quid.