No breach of the building regs procedures can be prosecuted after 6 months have elapsed, as the offence is non-inditable (i.e. Magistrates Court only). The same limitation restricts the absolute maximum fine to 5K, which is very unlikley to be imposed in any case, despite what some bodies who should know better have been saying..
So unless family and friends shop you in the first 6 months, you have got away with it.
A more serious prosecution can follow at any time if the installation causes accident, injury, or even death, but then that was true before Part P was introduced too, and will still apply even if the part P process has been followed to the letter.
More realistically, it may mean that when you sell your house, if it looks like un-notified work has been done that someone would like you to pay for a 'regularisation' or inspection of the wiring to show its condition, but this is probably as bad as it gets.
I can't imagine, given the cost of a house move, that an extra hundred quid will be that much of a concern if it gets between someone and their dream house, and it is probably cheaper done once than repeatedly calling out building control for every odd job. The secret of course is to do it well enough that if it is inspected, no remedial work is required.