I know around the east there is more rock and loose stone surface than your area Kris.
Brownie is the only person I know that runs a 21 and has ridden east a few times, and he has said he'd have a 43 for riding this area. If I road south Devon often I think I would also want a 21 or even something a bit more knobbly.
I'm not 100% on that though as the 43 is an acquired taste - once you get used to feathering the throttle (a little self restraint) you can get over the muddy stuff and then benefit from the increased grip for wet rocky stuff. Also I've never felt the need to run lower than 14 psi, I know some run much lower to improve it's traction.
MT43 ... too hard for my taste,I like Michelin Competition trials crossply rears with 10psi. It spins if too much throttle is used in mud but usually keeps me moving,sometimes with foot assistance.Never slips off wet rock unless using excessive power.Throttle restraint in the right gear will get you places no amount of power can reach.