I am an electrician, however when I wanted my mothers house rewiring I went to a firm of electricians, the main reason was time, what did reduce costs was I did all the plastering, with modern chasing machines not much point you doing that, and working where the home owner agreed to do work, only ever did it once, an electrician needs to get on with the job and the last thing he wants is to arrive and find agreed work not done, we ended up doing so much of the work the client had agreed to do that we may as well not have bothered allowing him to do anything. Once bitten twice shy, to finish off the work doing the plastering great, but any work which will delay the electrician if not done they will not want to know.
By getting a firm to do the work I saved on LABC fees, also some of my test gear is faulty and would need replacing so by time I took off the price of materials and LABC fees and new test equipment I was looking at paying less than £1000 for a weeks work by two electricians, it would have taken me at least a month more like two months to do myself so well worth it, specially as I only had 3 weeks before mother due home.
As to selecting as said personal recommendation is the best, all electricians will need to be a member of a scheme to be able to self certify, however if a member of any scheme to find them work like trusted traders or names to that effect, you really do need to ask why, most electricians can get enough work without paying money to people like them, only new electricians who don't have a good name will need them, so if a member of something to find them work, then steer clear of them.