From my experience of JG this is rubberised car-dashboard spray-paint on top of standard plastic, you'll want to remove this layer first with fine grade sandpaper (you'll need to buy a roll of it as the rubber ruins a sheet of sandpaper pretty quickly. I used a one grade of sandpaper to start, then a finer grade to smooth it all out once the majority removed). Putting paint straight on top of this rubber layer will cause the rubber to perish and come off, looking pretty shoddy.
Not sure what homebase/b&q sell, but Halfords sell 'camo' paint that's very good. Best bet would be ordering some Krylon Fusion paint @ about £7 a can from someone like Firesupport.
Give it thin coats and then leave it to dry for a week, (will be dry to touch after hour or two but not properly set for a week so don't use/touch it until that's passed for best results)
This will all be best done by taking the whole gun apart so you have just the stock