I posted a thread on RB4 which I think has aleady been linked to. For RB4 our route was:
Plymouth - Milford Haven
Milford Haven - Bangor NI*
Bangor - Kyle of Lochalsh
Kyle - Peterhead
Peterhead - Whitby
Whitby - Wells next the sea
Wells - Ramsgate
Ramsgate - Soton
Soton - Plymouth
(The official RB4 route went to Port St Mary IOM - we went to Bangor)
You could also contact Brian who did a circumnavigation last year.
If I were planning to do it again I think I would shorten the Kyle - Peterhead leg as this is a long one (c250nm) and could be bad weather etc. Maybe do astop on Scrabster at the very top of Scotland. Peterhead was a good spot and definately to be preferred to Fraserburgh.
On the East Coast, the other options would be Amble, Grimsby etc. I would avoid Wells, although its a great town the access in onshore weather would be tricky. (Lots of sandbanks, not much depth). Lowestoft is a good stop as would be Harwich (although you really want to run directly down from Lowestoft to Ramsgate outside the Thames banks.)
Assume you would do this in a petrol boat? The avalilability of waterside petrol will largely dictate where you stop either overnight or just for a refuel. Going around the coast, AFAIK petrol can be had at Falmouth, Milford Haven, Pywhelli, Bangor, Puffin Divers nr Oban, not sure about Kyle but garage isnt too far from the moorings. Not sure about the north east coast, definately not at Wells (in fact nearest garage is 10mile drive away!), Harwich yes, Ramsgate yes and you probably know the south coast better than me.
Anyway, theres some thought starters for you. Let me know if you want anymore info.