I don’t understand how this works for nominee accounts. What is to stop someone buying a share, obtaining such a letter from the broker, sending it to the company and then selling the share the next day. Shareholder perk entitlement can really only be verified when the company has the shareholder’s name on their own register. How else is the company to know when the person is no longer a shareholder? Do they rely on the broker to write to them when a person sells a holding?
If a company is willing to accept a broker’s letter about ownership at a given moment, with no way to know when the shareholding is sold subsequently, how about submitting the monthly statement that you receive from T212, or contract note showing the purchase, or even screenshot from trading app?
A quick search reveals that AJ Bell and Interactive Investor do not help with shareholder perks. I see that Hargreaves Lansdown does, and Freetrade mentions 7 UK companies, providing perks Carnival, L&G, Next, … and states that claiming involves writing to the company with proof of share ownership.