If I was doing that trip again I wouldn't do all those motorway miles, but bearing in mind the recent weather, the aim was to get back as quick as possible. From several comments it's clear that I missed some nice roads, though it looks hard to get past Manchester and Birmingham without using the M6. I did wonder about going over to the east and down those nice old A roads through Lincolnshire etc., but it's a hell of detour. On reflection, that kind of trip needs an alternative kind of UK road map, which clearly shows the good biking roads. Or maybe there already is such a map?
As for slow touring, there's more time to look around and that's great, but your attention can wander and it can get kind of mesmerising after a few hours. Because of that, I stopped for fuel and a break about every 100 miles - wow, a whole gallon of petrol, what an expense! I also realised I was getting dehydrated, which may have contributed to my state of mind.
I've now got some ordinary handlebars to replace the cruiser bars, and sent the bike's seat off to Viking Motorcycle Seats to be levelled out. I don't see why those US-type seats are so popular - surely you need to be able to move around the seat on a long trip? Unless you possess an armour-plated backside, of course.
Cheers, Martin