Firstly, Churchills near Lane End had a whole load of what you would consider beginner guns (Miroku, Browning, Beretta...) in their gun shop that are used for lessons which you can use/try out if you book a lesson or two and was gonna be the option I took before I saw what ended up being my 525 for sale, buuuut... secondly, most gun shops appear to let you borrow second hand guns to try out one at a time if there's a range nearby, as long as you have a valid shotgun or firearms certificate to show them, and probably some will want a returnable deposit for collateral or summat I'd have thought. Look on Guntrader & Gunstar would be my advice and go from there if there's guns you might actually buy... but probably advisable to not take the p1ss and just go shooting for fun, have a look at what you might ACTUALLY buy from a shop, not what you fancy having a jolly with. They're not daft!
Be amazed if you'll be able to try anything brand new though cos it knackers the resale for shops, obviously.
Can't speak highly enough of William Evans at Bisley (your area), but no idea what their policies on trying are. You might be allowed to take something to Long Siberia, the Pay & Play at Bisley, but you'll also need insurance to shoot there and it ain't cheap!