Big thanks to you all - thanks to the comments here my route has changed considerably, and I'm 100% sure it's for the better!
For anyone interested, this is what I'm doing. Anyone who will be in the same place as me at any point (this is all mid to late Aug), let me know!
(Remember, this is my first tour, and first time camping, so distances reflect this!)
Day 1 - Driven to Moffat, cycle west to Galloway Forest Park and surrounding area (inc Bladnoch distillery) (1.5 days)
Day 3 - Driven to Luss, cycle parts of the Trossachs and then head east to Stirling and surroundings (2 days).
Day 5 - Driven to Oban, Ferry to Mull, camp on Calgary Bay.
Day 6 - Ferry from Tobermory to Kilchoan (mainland). Cycle up to Mallaig (1.5 days)
Day 7 - Ferry over to Skye, days cycle ending in Portree
Day 8 - Northern part of Skye, ferry to Tarbet (Isle of Lewis), camp on a beach somewhere on Harris
Day 9 - Ferry from Stornoway to Ullapool, cycle down to Millarochy in Loch Lomond*** (3 or 4 days)
Day 13 - Leave the bike with the drivers and train to Edinburgh for Fringe Fest (3 days)
Day 16 - Evening train down to Kendal (Lake Disctrict)
Day 17 - Cycle around part of the Lakes
Day 18 - Cycle East for a route around the Dales
Day 19 - Driven back to Devon!
***Not sure whether to take the route down Loch Ness and the Caledonian Canal and down around Lomond, or whether to come down through Inverness and the Cairngorms. Ideally I want to do the Loch/Canal/Lomond route - after all it was seeing these that first inspired me to do this tour - but I'm scared of the midges around here, wherever I have to stop and camp! Suggestions welcome!