Hey Josh. Above any allowances that you have you will just need to complete a self assessment with HMRC. Its a bit of a pain but simple enough unless you have a complex investment plan.
Best ‘advice’ is generally to use an ISA. Its a convenient tax-wrapper for everyday people.
I’m curious as to how and why Trading 212 did tax you though? Did they just assume that your allowance had been completely used and therefore taxed you at the full rate? I wasn’t aware brokers could do this to pass this onto HRMC on your behalf, but I could easily be wrong.