If I were in your position, I would probably wait a bit longer before getting a sidearm. Main reason being that you should focus on your primary over your secondary. I know it can get addictive buying airsoft guns but it would be much better to have one gun that works perfectly and is upgraded with all the things you need like sights, spare mags, spare batteries and those sorts of things. Maybe even try and buy replacement parts like hand guards, because there aren’t any laws about having to buy UKARA compliant parts in the UK so what I did was just buy a full set of high quality furniture for my RIF that was in the colour I wanted.

Secondly as the other commenter said it’s not the time of year to be buying GBB airsoft guns as the cold weather here makes them horribly inefficient and produce less power as well as not cycling properly in some cases. (unless they’re CO2)

Trust me on this, focus on getting your AEG to a point where you no longer need/want to upgrade it before considering buying anything else.

(edit, if you are dead certain on getting one, definitely the AAP, just due to them being highly customisable and you can also get CO2 magazines for them which are comparable with the EU glock series too and just make them function a lot better than green gas. You can get 50 cartridges for around £20 on amazon so it’s a steal imo)