I am a M&S Elite+ member. A few years ago, I ordered this "celebration cake" for my +1, who is not a M&S member at all, as we were travelling on her birthday. There was no issue with the request being for a non-M&S passenger.
A very sizable and yummy chocolate mousse cake (different from above) was served. We had a big slice each and asked the crew to enjoy the rest...
(This was long-haul J class.)