In reply to Removed User:
Langavat has been on the radar for a while! Including a portage from the south first...
James;
More than one canoe sounds sensible, it adds a safety margin. 5-6 hours is also sensible. Paddling is generally a "slow and steady" thing on sheltered water, but if the wind finds you it can be tough. I plan on 2 1/2 mph when solo and not pushing hard. 3-4mph tandem. Obviously a river with flow will be faster.
Wind is your enemy in a canoe, bear that in mind with planning, sometimes you get "wind bound" on open water, less likely on rivers, but watch the levels.
The Wye is the obvious trip, and is really beautiful. Busy in season, but you'll be after the worst. Wild camping is probably frowned upon due to how busy it can be, so is sensitive. However the camp sites are generally excellent and very pleasant.
The Great Glen is an option, but the big lochs are very serious in wind, and autumn can be windy...a high failure rate at that time of year. There are other good trips in Scotland, (I'm heading up tomorrow!) but the weather can make it tough and wild lochs are not the best places for beginners when its windier than about 12-15mph (so a lot of the time!!!). The linked article includes some slightly smaller lochs too, which might be suitable. Hire that far from Inverness might cause problems though.
The Severn is like a slightly gentler Wye for a lot of the trip but has one harder rapid.
The Upper Thames (Lechlade to Oxford-ish) is surprisingly rural. Normally there are "lock camps" to choose from, but don't think they've re-opened yet. Wild camping should be possible done properly.
Song of the Paddle is the place to ask, or just look at the blogs section.