Practice the mantra, "It pre-exists my ownership of the property".

In reality, if the seller wants to sell, the buyer wants to buy and
the mortgage lender wants to lend - a way will be found. The system
will not be collapsing into chaos any time soon.

In practice Building Control don't do dawn raids to look at your
plumbing. All their time is taken up with preventing madmen from
creating fire-traps and self-collapsing buildings (and on simply
ploughing through all the applications and notices before them).

I would be much more concerned about planning violations coming back
to bite subsequent owners (or that previous building work was very
poorly executed rather than non-compliant).

Can you give us a link to the test case you mentioned?