I can just open an account online right? I have some experience in crypto, is it pretty much the same as just buying BTC, without needing your own personal/desktop wallet?
I run Linux (Ubuntu) on my laptop, is there a good desktop client (I have a good windows emulator installed if there isn't a Linux one) or is it all just done online?
Best site? Or is there just the one?
One more question (sorry for so many in one post) - if it's all online, why would it matter if I'm not in the USA? Or am i missing something

Thanks, btw
Right. Crypto is 100x more complicated since you have offline / online wallets. On top of exchanges (coinbase, binance, bittrexx, etc). Erase all that from your head since doesn't really apply to stocks.
Since you are in the UK -
Choosing the best broker can be crucial to your success as an active trader, as the needs of active traders can be drastically different from those of passive i
uk.stockbrokers.com
I use IBKR personally. You can trade in the browser and phone. But they also have trader workstation which is dope. Not really day trading these days like I used to but having trader workstation was a must to chart daily, checking different RSI, etc.
So for you simply open an account with one of those brokers. Deposit some money from your bank, and then buy. Best way is simply investing. UK index is FSTE 100. I would just buy that. Something very basic for you would be something like -
50% FSTE 100
50% bonds
Some more examples -
Portfolios should be thought as a team, where the whole is greater than the sum of the parts
www.sharesmagazine.co.uk
When the market shows a nice red day, buy the dip.
As you learn more you can start to get into individual stuff and break it down more.
25% FSTE 100
25% bonds
20% gold
10% silver
30% random shit
All the stuff that is going on now with reddit (gme, amc, etc) is kinda just nonsense and not normal. Someone is always going to get holding the bag this way. I would advise to stay away from day trading when you start or anything get rich quick. Never works. Long term investing / daily cost average is the way to go.
Of course investing in companies you believe in too. This is where the big gains usually happen.
Probably an easy way to get into the USA market also for you.
Lastly, tons and tons of information on youtube. This kid claytrades is on of my favorites and probably learned how to scalp (quick trades) mostly from him.
But also, at this point everything is super bloated and we can certainly head into a super crash. But nobody really knows. All the fundamentals of how the stock market should work have gone to hell since trump came (alongside the fact that everyone has a cell phone and can download a stock app and start trading in 10 minutes).