Any question answered...

Define road tyre. You should be OK on anything suitable for Paris-Roubaix

Continental Competition tubs.

signposting is okay, but surface varies dramatically and somewhat inconveniently since many of the routes suggest a longish ride, then suddenly go a bit off-road, down a sandy beach path, flinty road or bumpy drove.

if you don't mind a bit of off-road and have puncture-resistant tyres you should be okay, just be prepared for it. which are 20 and 21?

Ah. Bugger.

having said that sign-posting is okay, I would always take OS maps too, I've had to refer to them on occasion.

Time for plan C by the sounds of it.