I'm entering Borders of Belgium for next Sept.
Recommended. For me it turned into the Borders of Belgium 500k, but it has a lot going for it - no language problems most of the time cycle-friendly country (just knock on any door and ask for water, I did this quite a few times and there's no hesitation in filling your bottles). It's easy to get to, lots of places to stay nearby (we booked an AirBnB place for a few days before and afterwards). Very well organised. I did, and wrote about in Arrivee, a 600k in Belgium, which seems to to me to be the spiritual home of audax. I've ridden more k on the continent than in the UK in the past year or so. Living in the south-east makes this easier but so does being a member of the metropolitan liberal international elite, obvs.