Grasmere (Also Lake District)
https://grasmerehostel.co.uk/welcome
There's also a high concentration of YHAs in the Lakes
I'm in. http://ridewithgps.com/trips/2463713Taking @mcshroom's suggestion above for exclusive use for 24 people at £525 per night over 4 nights would work out at £2100 = £87.50 a skull (£22 pppn) for a weekend away in the Lakes!
Bike storage, drying room, en suite rooms, kitchens, pub nearby (Six Pints @Edwardoka @Harry_Palmer79 @DougieAB??) - even a sauna!! What's not to like?!?
Rothbury is a nice village. You'd have shops and cafes. No maximum security prisons but you do have the National trust property at Cragside if you fancy a day off the bike:
http://www.nationaltrust.org.uk/cragside/
This is a really nice loop (but hilly):
https://www.strava.com/activities/198146220
The nearest train stations to Rothbury are Berwick upon Tweed or Alnmouth (Alnmouth is also on the main East Coast line but does not get as many trains as Berwick does).
I'd cycle to Rothbury from Edinburgh via Duns, Coldstream and Wooler. I have cycled all these roads and would cycle them again.
It took me a while to locate Tarset Tor bunkhouse (which does look very nice)! It is between Kielder Water and Bellingham but only 3 miles out of Bellingham, and you have shops and pubs and stuff in Bellingham. Nearest train station is Hexham (18 miles away) - you'd need to go to Newcastle or Carlisle and change. Getting all the way there by bike? You could go over Carter Bar (which I have done, and the Fridays cycled the other way when they did LonJog) but it is a busy road and you'd have to accept that. There also appears to be a road that goes north from Kielder and over the border that way - I don't know that road. Going south from Bellingham you have lots of options for days out. It is all quite hilly!
South West Englandshire not allowed to chime in?
For me the format matters less, location is more of interest. From a purely selfish perspective I would of course love to see a place I did not visit before - Far North, Galloway, Orkney, Lake district...
Some places would perhaps present a certain logistical challenge, especially in the case of strong attendance - in this case the "meetup" format may be preferable.
Of course, youse are all also welcome to come down to Exmoor, and I can show you what real hills look like!
Rothbury loop ... I did that just the other day,Rothbury is a nice village. You'd have shops and cafes. No maximum security prisons but you do have the National trust property at Cragside if you fancy a day off the bike:
http://www.nationaltrust.org.uk/cragside/
This is a really nice loop (but hilly):
https://www.strava.com/activities/198146220
The nearest train stations to Rothbury are Berwick upon Tweed or Alnmouth (Alnmouth is also on the main East Coast line but does not get as many trains as Berwick does).
Nearest train station is Hexham (18 miles away) - you'd need to go to Newcastle or Carlisle and change. Getting all the way there by bike?
Rothbury loop ... I did that just the other day, but with a few changes that make it a tad hillier, but also a bit lovelier
Rothbury also has a bunkhouse that is proving popular: http://www.tomlinsonsrothbury.co.uk/
Trains: there's also Morpeth, on the Edinburgh/Newcastle/London route: the ride from Morpeth out to Rothbury, Bellingham, Kielder etc is lovely.