What Mr B said, for 500 pounds you can get a very good gun, you can get a very good one for even less. I started off with a budget of 450 euro's so around 370 pounds, and got everything I needed (oke it is a budget rifle I admit ^^"), so gear does not have to be expensive.
About the luggage, I go by public transport so I use a carrying bag for my gun, that isn't too obvious, if I had a car, I would have gotten a gun case, since when you arrive at the site you just drop your stuff somewhere
I think you should follow mr B's advice, around 500 pounds on the gun (and maybe attachments) and 400 left for the gear. Try to look around what you want, and make an excel sheet. You can get some good stuff in my opinion for that money.