The new version of "Approved Document P" which appeared in April 2013 introduced the concept of 'third party certification'- but no such scheme (for 'third party certifiers) has yet appeared - and probably never will.
To everyone's knowledge, although this is an option on paper, in practice there simply are no registered 3rd-party certifiers.
It seems that we are behind the times. Listed here are 2 organisations who run such schemes, NAPIT & Stroma.
The Stroma site seems to be lacking anything as useful as a 'find a member' search, but I tried NAPIT's site, and looked for ones in London, and it found 6.

Tried a few other locations - coverage seems a bit patchy, e.g. 10 in Birmingham, 5 in Leeds, but only 1 in Manchester, 1 in Liverpool & 1 in Newcastle.