as others have mentioned it's important to be clear on what the budget includes.
is it £300 for just the gat, or gat, mags, batteries, charger, bag of bb's, eyepro, webbing etc is gonna make a massive difference to what's an appropriate recommendation for us to make.
the other kit you have can be just as important/useful as your pew, for example if you're lacking a really good pair of boots and want to play outdoors then investing £100+ in a good solid pair is going to do more for increasing your in-game enjoyment than the same amount spent on a pew.