Eugene's tip about guns on pegs is a great place to look.
The first thing to decide is what sort of shooting you want to purchase. Driven can get expensive, with a lot of the famous shoots charging 40GBP+ per bird. That makes a 400 bird day rather pricey, even for a line of 8 guns. Fortunately there's a lot of other, well-managed and challenging shooting available that doesn't require that sort of expenditure. If your dates work out, Dig Haddoke's shoots are a bargain and very well run, even if you'll be expected to pay for a fair number of rounds of bitter after the shoot! There are a number of packages offered by various agents that include driven, walked-up and, sometimes, stalking. If you search around, you'll find some good ones that suit your budget. You'll also get a warm welcome from all participants, whether blokes like us or peers of the realm. They're all there to shoot!
By the way, Eugene, I missed Wales last year, but I'm back in December. The Lion for dinner?