Firstly, after you have planning permission the role of the planners seems to be complete. There is no inspection to confirm that whatever you have done conforms to the plans submitted.
Building Regulations is a separate department - in fact a separate business - and has no links to planning at all. They don't enforce planning, just BR.
So the planners have no idea if the plans have been followed unless someone (irate neighbour, usually) registers a complaint.
Secondly, BR do not (certainly in our case) do a final inspection to confirm that all works have been completed to BR and all gas and electrical work has been certified.
We know this because our certificate of completion turned up in the post today - without anyone coming round to check that we had done everything by the book.
Now in our case we did ensure regular inspections at key stages of the build and we did follow the plans.
However if we hadn't, then who would know?
We have planning permission and BR sign off.
Makes you wonder how much all this process is worth, really.