So, apart from what the Regulations and Rules and Guidelines say and the fact that we have an Army of disconnected riders/Organisers, interpreting these as it suits them, what are the ‘Real World’ implications of non-notification in the event of needing the AUK Insurance (verses BC or CTC Insurance which offers undated cover).
BC/CTC insurance offers third party insurance for the rider only. AIUI Audax UK's rider/event insurance covers more than this.
For undated rides BC/CTC insurance won't provide any cover for the DIY organiser or Audax UK should someone (third party or the rider themselves) come up with some reason why they would sue them because of a DIY/Perm rider's actions.
Granted, I can't think of a reason why someone would be able to successfully sue Audax UK or the DIY organiser for the actions of a rider on a DIY, but if insurance is there it can be used to help defend against unnecessary claims. A perm is slightly different, a Perm organiser has more responsibility given that they are providing a routesheet and information. Still it's unlikely (but not impossible).
If AUK, or the organiser, want to avoid this by saying "we're not responsible as the rider hadn't entered the event properly by not providing me with an intended ride date" then all such Brevets (that occur without accident) should be refused validation.
The other point is about stating what you are trying to achieve before doing it.
Take the LEL route and create a series of DIYs such as:-
London, Gamlingay
London, Gamlingay, London
London, Gamlingay, Thurlby
London, Gamlingay, Thurlby, Gamlingay
London, Gamlingay, Thurlby, Gamlingay, London
London, Gamlingay, Thurlby, Lincoln
London, Gamlingay, Thurlby, Lincoln, Thurlby
London, Gamlingay, Thurlby, Lincoln, Thurlby, Gamlingay
...etc...
Now you can just set off from London and see how far you get. You've theoretically entered all of these DIYs in advance (by getting the distances confirmed and buying a multi-pack of DIY cards) so why bother telling anyone which one you're actually doing? You may set off intending to just do a one way ride to Lincoln for 200km, but you're feeling strong when you get to Lincoln so you decide to turn back and ride all the way back to London, only to get bored at Gamlingay and call it a day and put in a claim for a 300km ride.
Now compare that to someone who enters London to Edinburgh and back but packs at Gamlingay on the way back due to illness. 1350km done and 0 points. No chance of claiming anything.
Doesn't apply quite so much to Perms but I'm sure there are some with common starting points and common controls.