I opened a stocks and shares ISA recently on Trading 212. So far I think it's a great app - the thing that first attracted me was that there are no platform charges (most of the alternatives charge a % of your investments just to hold them) and no fees for buying and selling, so it's very good value especially for small amounts.
It's a UK company and regulated by the FCA. All the reviews I've seen have been excellent too - seems like a great company. Everything is explained very clearly too.
If you sign up using my referral link, we each get a free share worth up to £100. Most of the free shares are under £25, but there's a 1/5 chance of getting one worth over £25. I got a €10 share in Orange - hoping to be luckier next time!
Last thing - they also offer a CFD account, which is basically gambling on share price movements. I'd definitely avoid that unless you really know what you are doing and stick to sensible investments like shares in established companies and funds. So stick to either the ISA or the standard invest account.
Link: https://www.trading212.com/invite/16bjA3Ceso