Several towns/cities in the UK have been mentioned as being relatively cycle friendly.
This information is really useful when planning a mini-tour; I'd rather go somewhere reasonably friendly than somewhere hostile.
Milton Keynes, but that always has to be tinged with sadness for me because the plentiful "Redways" cycling infrastructure is old, crumbling, often misrepresented (it was always a third-class network, giving way to mini-motorways and bus routes, arguably even canals and rivers) and never really achieved its potential. The city is basically cycle-friendly but it's even friendlier to cars.
A tour needs more than a town or city, though. That's what really discourages visiting Milton Keynes on a tour, as Bucks and Beds are poor and Northants worse, although there is the Varsity Way (Oxford to Cambridge, which maybe should be an ideal cycle-friendliness tour in theory) but that has been disrupted by HS2 and East West Rail construction works. While Cambridge may be nice, some of the bits around it are absolutely awful, but you could pick a lovely way out along the former railway to St Ives and then low-traffic routes to and through Huntingdon to get far enough away from the madness. There's also a route far enough alongside the A10 to get away and then the B road to Puckeridge and back roads through either Cold Christmas or High Cross to Ware to pick up the Lea Valley routes into London.
Bristol to Taunton would be a good ride and the route in between is finally showing signs of coming together: ride up the Avon gorge and cross to Portbury, then back roads (the Avon Cycleway formerly NCN 410, or the Down Road if you don't mind the short connecting B road or detour) to Clevedon, the new route 33 to/through WsM to Brean, Burnham, Bridgwater (all patchy for cycling) and picking up route 3 to Taunton. Bristol to Bath is a famous route and then it's possible to head south on the Two Tunnels route 24 but I'm not sure where after that. One day they might link it to a decent route through Wells to connect with an extended Strawberry Line (NCN 26) to Cheddar and Yatton and the Festival Way (another part of 33) to complete a loop back into Bristol but Somerset remains rather patchy.
For tours, I'll also mention the Norfolk loop tour I took a few years ago
https://cycle.travel/route/norfolk but that doesn't actually visit Norwich itself (Cycling UK's Rebellion Way does, but I'm not much for bridleway riding) and the joke around here is that people cycle despite the council infrastructure rather than because of it, so I'm not sure I can really recommend my area as an example of cycle-friendliness! Most drivers are nice to me, though. Still get the occasional rager but less than I've had elsewhere.