Ah, the classic HPA vs AEG debate...
Personally, especially for a first gun, I'd just go with an AEG. HPA setups are really expensive as you're either paying £800 for an MTW, or you're paying £400+ for a HPA engine to stick into an AEG, so it's probably going to end up about £600 there with no guarantee it will work amazingly well. Then you need a tank, line and regulator so that's another £100+ to add in. I'd much rather pick up an AEG plus a ML hop rubber and nub for a fraction of the price. When people say that they have a 1fps variance on their HPA setups, firstly it's more than possible to do that with AEGs (probably for cheaper), and secondly that level of consistency is entirely unnecessary; not saying people are wrong for chasing it, but you absolutely don't need it to be competitive and hit consistent, accurate shots. At the end of the day, we're shooting plastic balls at each other that weigh less than half a gram and all the consistency in the world won't stop Mother Nature from deciding that your shots shouldn't hit at times. Trust me, as a long time bolt action rifle user I've seen more than a few shots veer off target because of a rogue twig or gust of wind
£500 for a first gun is a lot, so you should be able to get something really nice. If you're interested in AR-15 SPR style rifles, I'd advise looking at the Cyma Platinum range. My friend has a cm097, which is an AR-15 SPR style rifle, and it's really good out of the box. Makes a fantastic platform to work on, but doesn't actually need that much to make it shoot really well. A Maple Leaf macaron hop rubber and an omega nub is all it really needs, but a nice 6.02 ZCI barrel can help with voluming and would easily fit into your budget. I'd spend the rest on load bearing gear, a nice sling, an optic, mags, an Odin speedloader, batteries, BBs etc. There's plenty of other things that you could get to make your life easier beyond just the gun, and this is all assuming you already have spent out on good eye protection and a good set of boots.
If you do eventually go down the GBBR route (a great route to go down, but definitely not for everyone), it'll still be great to have a solid AEG as a backup rifle, just in case of issues or cold winter temperatures making gas far less efficient.