A route check would need to be within two weeks of the calendar ride for the rider to be validated as a rider on the event (as far as I’m aware anyway).
You could always ride it in April, but it’d have to be a DIY perm in that case.
Is there any advantage to doing it within the 2 week window? What does being validated on the event get me compared to riding it, say, 3 weeks out. Would it then allow the 1000k to count towards Brevet 10000-esque awards or similar?
Advantage: The ten points would be calendar events points not DIY points, depending on your remotest interest in 'points'
AUK Reg 7.2.1 "Further Event Classifications:(b) Organisers of calendar events and their helpers may ride their own event(s) up to 15 days in advance or 7 days after the scheduled event date and be included in the event results."
Yes - if you ride the calendar (BRM) event route in support of the organiser (eg route check) then your ride can be homologated as a BRM, which one could count towards (say) ACP10k award. A DIY would only be BR, which would be fine for all the AUK Brevet type awards (and others) but couldn't be used for the ACP ones (5k and 10k).
HTH