Hargreaves Lansdown and IG are probably the best trading platforms-IG I find has a very good mobile app if that’s how you want to trade.
Beware of so called ‘experts’. Choose your own method of buying shares. However I always find Robbie Burns blogs/Naked Trader/Proactive investors site has useleful info.
You won’t pay tax on stocks & shares ISA however there will be stamp duty and fee/transaction.
Don’t get suckered into Spread betting (for now), it’s gambling in effect. I occasionally do it to short stock and make a quick couple of tax free quid. I have also lost a few quid!
And yes, you can trade, but at low volumes you’ll be incurring fees per transaction and probably not worth the effort. Be aware also that it’s £20k in s&s ISA-anything above that will have to be in a separate trading account that will be taxable.