Do you actually mean a round trip - Launch at Carlisle & haul back out at Berqwick kind of thing? There are occasional "round Britain" trips - someone is planning one in an SR4, have a search through the cruises posts.
Plenty of people do weeks & fortnights all round the coast, so could keep an eye open for them & join in, towing between bases. E.G. Dougcrock always has a week on the Clyde, Tony Hill seems to have made the Easdale weekend an annual feature.....
If you are seriously meaning round Scotland, the biggest problem might be getting from one end of the border to the other, as Hadrian built a wall and not a canal
. My suggestion if you really do mean round Scotland and not Britain would be to find a marina / quay near Berwick / Newastle & ditto somewhere near Carlisle. (a post and the locals will be able to give better advice) There is a regular train service between Newcastle & Carlisle (and reasonable connections between Berwick & Newcastle - you can then get back to your car / trailer with relative ease with a couple of short taxi rides either end
Other thing I guess with a 6.3m O-Pro is that round the top is maybe going to be a bit more likely than if you had a 3m SIB.
There's a few guys from up that neck of the woods will be able to give better advice on the area / places to stop / ake shelter, but always keep in mind if the weather really turns, and you have a deadline (e.g. ferry back) keep the Caley canal as a "plan B". One day passage with a rib (you have to keep to the speed limit in the "canal" sections but can open up on the lochs)
Also look at posts in the racing section about the Round Britain races.
Hope this helps.