emerges from the dark shadows
I actually went through this with Whitbread, not M&B, but yes the company administering the benefit card on their behalf did accept a note from Trading 212, and again with another note to renew it for 2021/22. I asked support to produce a confirmation of my holding in Whitbread, which arrived very quickly as a PDF only showing the relevant holding for Whitbread and branded with Trading 212’s logo, company details, etc.
Again, your mileage may vary depending on how strict the company are. If I were that way inclined, there is in theory nothing stopping me from purchasing the required 64 shares, getting a confirmation of holding, getting my benefit card which is good for 1 year, then immediately selling the shares again.