It's based on you can't ride on motorways but everywhere else is fair game.
There's a lot of local knowledge involved too. I think there's a road around Telford, for instance, which is, apparently, on an obvious path for people riding Lands End to John O'Groats but it's a death trap which locals all avoid. The A3 from Roehampton around Kingston to Cobham is a fast 3-lane carriageway which some enjoy riding but others wouldn't venture anywhere near. It is perfectly legal to cycle on the A322 where it is a dual-cabbageway from Slough to Windsor but, for some reason, drivers on that road seem to lose any sense of decency towards two-wheelers and I have never had one person move to the right to pass me when I'm on a bike - which is pretty rough as a few kids use it to cycle to school.
There are no roads that I wouldn't ride on - even the A404 where it passes over the M40 (which scares many people in cars!) - but there are certainly roads that I don't enjoy riding.
There's a loop from me which I seem to be doing regularly. It used to be on my commute and, at 8am, it's full of German "executive" cars doing 75+ and leaving little margin for error. On the other hand, after 9pm it is pretty much completely free of traffic. I've done it a few times recently and not had a car pass me in 30 minutes of riding which, given we're a few miles from the M25, M3 and M4, is pretty amazing.
