Welcome. First I think I should mention that there's no such thing as an airsoft licence. Site membership is what the ukara database uses, but this is not a legal requirement and other defences are available. Whether a shop or seller will accept them though is a different matter.
Have a look here
https://playairsoft.uk/
Costs will vary from site to site but usually you're looking at £20 to £35 for your game fees. Renting a gun for the day will cost more but usually comes with face protection and some bbs.
To get site membership you need to play 3 game days in a space of more than 2 months but less than 12. While there are other defences available not all shops accept them and quite a few are an absolute joke.
Most people will suggest renting first as it gives you an idea what's suitable for your needs. You don't want to spend a sizeable chunk of cash on something which is crap or you don't get on with. Yes you could sell it on but I can guarantee you won't make your money back, even more so as the gun would be a 2 tone
As others have posted, pretty much all questions you'll have have already been asked so read as much as you can on here. Personally I'd avoid advice from reddit and facebook as 99% of it is people confusing fact with opinion or just plain wrong