I’m in the UK Too and both DIY and prebuilt options are everywhere here.
If you have the time, I think DIY Is an amazing option. I’ve personally ridden through Bristol multiple times and the police are very friendly there and don’t mind you riding unless you’re being obviously obstructive to pedestrians/road users. Ride safely and they’ll be okay with you.
The prebuilt options you’ve got can be a bit underwhelming, while they’ll get the job done. Exway has poor range and on UK roads can be a really rough ride, and is really hard to do maintainance on as you have to take up the griptape for it, while the other budget belt options leave no real room for upgrades down the line, and if you do choose to make upgrades, you’ll end up scrapping large parts of the board.
DIY Is as easy to maintain as you’d like (My small commuter uses a simple wooden insert system with retaining bars, so I can open up the enclosures mid ride in a couple minutes.) You can also always grab a larger enclosure than you need and buy a larger battery or different ESC down the line.
At $600 you have plenty of options, I think I would tend towards a single 6374, a deck which can take larger wheels and either a 10S3P 30Q battery or some lipos as you’ve been looking at here. Flipsky ESCs can be okay, I personally have had mixed experiences with them. I had an old FSESC 6.6 Which blew after a year, and a recent 6.6 Mini blew after a week. With a distinct lack of options though, especially at 600 they’ll get you rolling in a pinch.
I would try to keep with larger wheels and flexy decks for ride comfort, although as wheels get larger I really personally love a good concaved stiff deck, example landyachtz switchblade or even an evo.
As for options for longboard trucks, our priced on calibers SUCK. They’re 3x as expensive as the US. You can mount to paris trucks with dickyhos, but both caliber’s and paris trucks won’t give you many options when it comes to pulley types and wheel sizes. If I were you, I’d go with Dickyho’s wide trucks, torqueboards 218mm or even evolve’s trucks