That looks pretty good to me Louis! I can't add anything to what the guys have said about painting/washes etc. but one fairly easy thing you could do is drill out the end of the gun barrel. I think it improves the look a lot for a small effort (you don't need to drill the whole length - just a millimetre or so just to show the barrel isn't solid).
A set of micro drills and pin vice will do the job, and you'll find yourself after a couple of months wondering how the h**l you managed without them!
For a first build you've done a great job. These little Airfix kits are a cheap and gentle introduction to model building, I hope it won't be your last.