The problem lies (and I know this through my own experience) that shooting is very hard to find; be that rough/vermin/sporting rights.
Most farms that have a shoot attached or let out, very very rarely let anyone else shoot over the land. Likewise, most shoots on farm land tend to keep the vermin shooting "in house" as a perk for the beaters (I do exactly this on my shoot)

The issue is, if you were a farm owner, would you allow someone you dont know lose on your land, even with an air gun?

My advice is thus: find a local shoot and offer your services (NOT to shoot vermin, dont even mention it to start with) some shoot owners & keepers will jump at the chance of free help, some may not. This way, you get to know the keeper/owner, they get to know you, you learn A LOT and you will then find yourself being offered the odd bit of sport. For a few hours work, the rewards are worth it.

Dont be affraid to ask, but be prepared for both knock-backs and the worst jobs going! If your keen, reliable and honest, then doors will open.

Good Luck
Cpt.