Personally I think £20k plus is rip off pricing, not doubt because they know roofers are in high demand.
Pre covid, my mother had a detached bungalow re-roofed, yes, not as much scaffolding, but it had a big footprint so probably as much materials as a typical detached house. They charged £7k including the (lesser) scaffolding, portaloo in the drive, etc - everything removed and replaced including batons, felt, etc.
I think tradesmen are just using rip-off pricing whilst they know they can get away with it.
Being cheap doesn't always mean being poor quality, shoddy or a con man.
When we had our back porch replaced, we got quotes ranging from £2.5k to £10k, and chose a supposedly reputable firm who charged somewhere in the middle. (The cheap guy was someone we'd used previously for some guttering replacements and seemed ok although he wasn't officially a PVC fitter). It was a nightmare, the "owner" never came near, and just sent a few random subbies who broke some floor tiles in the porch, put in the wrong width of window ledge, put up guttering that leaked, and the guy himself fouled up as we said "like for like" but the bottom of the door was glazed instead of white, the hinges were bright silver instead of white, etc. We weren't impressed at all.
A couple of years later we had the front porch replaced and used the cheap £2.5k guy (he came in cheapest again!). He did a brilliant job. Worked on his own, real attention to detail, did exactly what we said we wanted, no breakages, etc and we were really happy!
So, cost isn't really any indicator of quality.