If you want to get a permission you'll need to do the following:
1. Get BASC shooting insurance - It's not expensive and reassures land owners you are taking things seriously - it covers you if you have an accident and shoot someone/thing, it also means you get less hassle from the plod when they turn up.
2. Leg work - you'll have to approach a lot of land owners to get a permission - I had 12 knock backs before I found someone who would let me shoot on their land.
3. Dress smartly - don't turn up in a t-shirt and jeans if you are dressed smart casual you have a better chance, be polite explain you are willing to help out and that you'd be doing the land owner a favour.
4. Take your gun and get it Chronographed to make sure it's under the legal limit (12ft/lb) and more importantly powerful enough to kill with.
Couple of other pointers as well - read up on current VCR Law, and also what you can legally shoot and the reasons behind shooting them, you don't automatically have the right to shoot vermin even if you have persmission - it just take one eco mentalist to spoil your day.
If/When you get a permission you'll need a written and signed letter of authorisation from the land owner. PM me and I'll send you some useful links.
There is a gun club at Horton Grange - Near Dinnington Village where you can practise to your hearts content with for a fiver a day.
Good Luck.
Oh - You'd better read up on current gun laws in the UK as well they changed in February again - you don't want a visit from the boys in black carrying real guns