Just caught up with this... not been on for a while! Thanks for the comments on the vid.
Yes - the towpaths you have to slow down for. Although, the section I rode around Woking didn't seem to be especially busy, and in good enough order for my road bike. Although, I was riding on weekdays which I guess made a difference in terms of how many people were using them.
As for the A30, I didn't ride it. I've been down it on the motorbike though, and I'd agree it's probably a route I'd stay away from unless absolutely necessary. However, riding across Dartmoor was quite a nice alternative. Yes, it's got a lot more climbing. But the roads themselves generally seemed to be in a good state of repair and fine for road cycling. The sharpest climbs didn't seem to go on for too long either. The worst bit is actually coming out of Bovey Tracey up onto Dartmoor. It's about 4 miles I think, and seemed for the most part to be a consistent 7-10% gradient. Once you're up there though, it's not so bad. And the drop back down to Tavistock seemed to go on for miles, on relatively good tarmac.
There were only one or two very short sections of my overall route that I wouldn't generally consider any good for a road bike, where I had to drop down to maybe 10mph or less. The sections weren't long enough to make it worth redoing those parts of the route for though. The rest was fine, and while I used some A roads they were not horribly busy or fast. In fact I was pleasantly surprised by just how much room I was given by vehicles as I came out of Tavistock along the A390, despite grinding out of the Tamar valley at about 8mph.
In any case, if the OP does do the ride, I hope it goes well - there's plenty to see on the way down, and if it's anything like my ride was it'll make for some nice stories in cycling related discussions for years afterwards!