It's all a consequence of Audax UK inventing its own slight variation of the rules rather than taking the BRM standard from the beginning (and keeping in track with the associated changes). [Although I'm sure there's a lot more to it than this throwaway comment.]
There are quirks of this all over the place. If you take a theoretical 200 that is under 202.5km then you actually get more time if it were a BRM than if it were a BR (as long as the organiser sets the minimum average speed at 15kph rather than 14.3kph, which many organisers do). A BR 202km ride would be 13h28m with a 15kph min average speed and a BRM 200 is defined as having a time limit of 13h30m. (This works for 400km BRMs under 405km since the time limit is double that of a 200.)
BRMs have limits to the amount of over-distance they can have (which isn't always respected I note) but the on-road distance of the ride should be available in advance and the rider can do their own calculations and make their own mind up. Adverse weather doesn't automatically get you extra time but the possibility of it should be factored in by the rider.
I personally like the "you attempt the event that you enter" philosophy as I'm sometimes a lazy rider and will use all of the time available to me, if I knew I could relax a bit more and catch the BR timings instead of BRM (and I didn't need the BRM) I'd probably end up slacking off and coming in close to the BR timings too. (The one time I did the BCM I finished in >40h but luckily it was a BR year.)
I'd be happy with organisers offering both BR and BRM versions of an event running concurrently, but they would be unique events and you enter and attempt to complete just one of them - no upgrading/downgrading based on eventual results.
My point of a view is from that of an occasional audax rider who enters local rides for enjoyment and longer, harder rides for the challenge as a standalone event.
Why are you worried about validation then? You finished the ride but just happened to be outside the time limits. It's still a great achievement. As someone else said, stick it on Strava and bathe in the kudos.
If you want a validated ride then you need to finish within the time limit of the ride that was known in advance.