The Department for Communities and Local Government (along with some land-related company) say "if a person carrying out building work contravenes the Building Regulations, the local authority may prosecute them in the Magistrates' Court where an unlimited fine may be imposed (sections 35 and 35A of the Building Act 1984). Prosecution is possible up to two years after the completion of the offending work. This action will usually be taken against the person carrying out the work (builder, installer or main contractor)". The other thing they say is "Alternatively, or in addition, the local authority may serve an enforcement notice on the building owner requiring alteration or removal of work which contravenes the regulations (section 36 of the 1984 Act). If the owner does not comply with the notice the local authority has the power to undertake the work itself and recover the costs of doing so from the owner". So the problem has to be fixed; but the penalties against the builder would be greater. There is a 1 year limit on enforcement against the owner, and if violation was not specifically rejected, a section 36 ("against the owner") enforcement is precluded.
What's a little unclear here is why the inspectors did not catch the violation. I assume there is a completion certificate, which came to light with the land search For example, maybe it is not technically a violation. It seems that if you hire a "competent person" (a registered tradesperson), the builder can self-certify that the work was done to standard, but they should have insurance and formal procedures for addressing complaints.
Enforcement seems to be in the hands of the local government.