1) stop - don't go shopping yet
2) check out the map :
https://mapsengine.google.com/map/edit?mid=zdRfWqkdmKGw.kbddocq_JqUY
It does not show everywhere, but is a start
3) I can recommend Holmbush & Campaign Cobham
And for shopping you can visit Lips at Welwyn Garden City - but as at (1) don't go shopping yet
4) look around at what kinds of paintball there are.
You will have played punter paintball - this is the most expensive way to play, as it has everything supplied, has all the maintenance, cleaning and staff costs for the site and is for irregular or once only players. This is also the bread and butter of the industry. Because the infrastructure is there for the punter, we (the own gunners / regulars) can turn up and use our own gear - there are some lesser costs to the site and we can be treated as regulars.
Depending on what interests you there are:
Walkons - call the sites near you and ask about walkons. These will probably be held once a month. How it is at a specific site is dependant on the local regular crowd. Walkons are often very intense and can be a shock when moving from punter to walkon, however the crowd will be friendly & welcoming
(If not then walk away and pick a different site)
Next up are events, such as scenarios / big games. These happen across the country, mostly about one per year for specific sites. These are my favourite type of paintball
For all of the above you can play as an individual, and either stay that way or make new paintballing friends (it's very hard to not make new paintballing friends)
Another option is speedball - you pretty much need a team, but it is possible to tag on with some on occasion. This is the competitive tournament style
5) you can often play walkons and events by renting gear - this gives you the chance to experience more paintball - and more importantly check out what is around, speak to people about gear, try out other people's guns etc
6) spending your £300
As above hold back on spending, you can make it go further by buying secondhand, but also don't need everything straight away, so you could buy something early on then wait for each payday
Nothing has to be top of the range
Priority goes to goggles
You will need a gun
Both of the above are down to personal preference
To go with them you need a hopper, air system (a 3000psi bottle is fine at £30/£40 new), pods for reloading in game and a pod belt or vest to carry the
Finally clothes and footwear - you don't need flash paintball specific gear. Comfy clothing is sufficient