I don't consider myself an investor, mainly as I don't have sufficient disposable income to invest. However over the years I've accumulated a number of share certificates for various public companies. All of these companies are still trading. Most certificates are for small amounts (e.g. 30 shares of low-value stock), others are for larger amounts (several thousand shares each).
Recently I started looking for an online brokerage to deposit these certificate shares - so that I could sell them when the time is right. I am looking for a cheap trading platform, with online access - which is specifically cheap for low-volume trades (e.g. I may end up doing one trade a year). I want to avoid paying maintenance and only pay for trade execution.
Searching online produces some results that seem dodgy, scammy or otherwise not worthy my attention, while some results seem genuine. Considering I am in the UK, how do I validate that an online-only trading platform is genuine and not a scam?
Please note, that I am not asking for recommendations but rather how to validate specific references I already have.