An easy 400, 500 or 600? Well you've missed it.
I failed on riding a 400 a few years ago when sleep got the better of me, so I looked for an easy replacement 400. I settled on the 24 hour Time Trial as AUK will award you points depending on the kilometres completed. For me, 500km is an achievable target within 24 hours - only just - but 600km definitely not. As I was only aiming for 400km, I was able to take it leisurely. I even had a nap during the night on my way to 440km.
If you can get to the start, the logistics are easy, with marshals pointing the way, with refreshments available at the 24 hour petrol station, and (in those days) with the Raven Café open for 24 hours. The route might be a bit monotonous, but if you don't go eye-balls out and just aim for 400km, you don't get to do as many laps.
But as I say, that boat has sailed this year. You also mentioned AAA points, which aren't available. It doesn't count as a BRM so has no value for PBP.