Trading 212 have just restarted their promotion where you can receive a free share (worth €8 - €100, about £6.75 - £84.40) for opening an investment or stocks and shares ISA account with a referral link. Minimum investment is £1, but you do need to actually use the account for a while so you can't just sign up and withdraw straight away.
They are currently paying 5.2% interest on uninvested cash in the account, which is one of the best savings rates you can get anywhere at the moment so it's actually worth opening an account just for that. You need to make sure you enable interest manually as it's off by default.
Also a whole range of shares and funds you can invest in, including low cost index tracking funds which are one of the best ways to invest long term. Unlike most investment platforms, there are no charges for buying and selling UK shares and funds and no annual charge.
Referral link: https://www.trading212.com/invite/16bjA3Ceso