I think Innerleithen/Tweed Valley would be an excellent choice of control to base a holiday around. There's lots of walking and cycling in the vicinity on a variety of surfaces, and Edinburgh and the East Lothian coast are short distances away if you want to do day trips.
Innerleithen will be at the 840km-ish mark, so at 12kmh/30kmh limits, riders will have 42 hour windows to pass through. Allowing for spread of start times, it should be 'active' for 55 hours or so. Of course, the controller will need volunteers before and after to set up/dismantle the control.