Aye GW tend to be pretty good with stuff they make if it's obviously faulty, at least for the cheap stuff, for things like paints a GW store probably won't even blink at swapping out if the cap is obviously a factory defect regardless of if you've got the receipt on you.
As wildman says avoid cheap airbrushes/compressors if you can, they can work out well but a lot of the time the quality control isn't there and it's hard to get help or spares
I've seen so many people say how they tried cheap brushes and basically gave up on the idea for years because they had all sorts of "little" problems they couldn't fix or the results were always poor.
A cheap "good" brush where you can get all the parts with relative ease can be had from the likes of Harder & Steinbeck from around £60, with their Evolution 2 in 1 sets (two needle/nozzle sets and two paint cups) which are great for both smaller models and scenery from about £130.
As for compressors always try to go for one with a tank unless space is at a major premium, it means it'll always hit the chosen pressure straight from the moment you start using the brush, and avoids the possibility of "pulsing" in the air supply where there is not sufficient air in the line to even out the flow when the compressor cycles between strokes (you can overcome it by a longer airline, or IIRC Iwata use the handle in one of their portable models to act as a small smoothing tank).
Depending on where you are, always keep an eye on ebay, the likes of tattoo parlors, nail salons and dentists often put their old compressors on ebay as "collect only" due to weight, and the likes of Bambi and Jun-air models which are somewhere between a garage compressor and "airbrush" compressor can pop up really cheap. Both companies make compressors intended for what is effectively retail floor/customer facing use* so they tend to be both very quiet like a hobby airbrush compressor, and well made as they're expected to keep going for years of often heavy daily use.
I still regret a bit not bidding higher on a bambi compressor that went for ~£150 near me a few years back, especially as it came with machine that let you assemble picture frames.
*They're basically silent/medical versions of small garage compressors.