I think many of us know that. But that legislation doesn’t actually define the minimum service that is just enough to avoid closure action, which was the subject in question...
I tried tracing the legislation back to the Transport Act 1962 and various Railways Act from the Victorian era, and can't see anything. My guess is it's an "inertia" thing. Someone, somewhere, sitting in an office in the British Rail headquarters probably had a reason for determining that services should be run once every 'x' days in one direction. It possibly came out of court challenges to closures.
That memo then got passed down to the TOCs / DfT / ORR / Whoever and it's just kind of been followed ever since.
Except, a bit of research shows it hasn't been followed, in fact. On the ORR website, they've issued a Closure notice for Newhaven Marine station. The consultation period was from January to April 2020.
Only problem is, the last train ran there in 2006 and they demolished the station (apart from one platform face) in 2017!! Despite this, nobody seems to have realised they needed to run a "Parliamentary" service, or go through a closure notice until last year! And nothing seemed to happen. I'd be interested to know what would happen if they
refused to close a line? Has that ever occurred in recent years?