I have just done the same thing.
Two choices.
1. Get the work done by a part P registered electrician.
2. Put in a building notice with the council, get an electrician to do the work or do it yourself, then get the installation tested by a competent person. (Normally this would be a qualified electrician but in this case they DO NOT have to be part P registered)
I went route 2, did the work myself then got it tested but I had to put in a building notice for some construction work anyway.
Good luck