Strictly speaking it is true that its not the builders responsibility to issue the Electrical Installation Certificate and register the job online as required by part P of the building regulations, as this duty rests with the electrician who does the work.
Often however builders want to use the cheapest labour they can get, and will knowingly hire un-registered electricians who can't issue the paperwork or register the job! So the crux of the matter is really is the electrician registered or not? If he is then you should write to him stating that you will be making a complaint to his registered scheme provider (NICEIC, NAPIT, ECA, etc) they will investigate your complaint and help resolve it for free.
Of course if he is un-registered (highly likely) then you need to cliam compensation from the builder and get him to pay for local building control to come out and inspect the works etc. If any defects are found then really he should be liable for the costs of putting these right as well.
To anyone else reading this, do yourself a favour and hire a registered electrician directly, do not use builders to source labour as 95% of the time you pay a higher price as the builder puts a mark up on the electricians work and the people they hire are seldom registered electricians, so you get a poorer quality job and no paperwork!