The thing is @ArmitageShanked I don't think that Sunup had the slightest idea what the song was referring to until someone else explained it! If I am correct, then s/he was probably embarrassed at not knowing, so went on the defensive, which made her annoyed.
I haven't seen you say anywhere that you were actually offended by the advert. I thought you meant that you thought it was in poor taste to have used that whole episode in time as a frivolous advertisement, and I agree with you.
I was also surprised at MrsTerryPratchett's first reply to you, it didn't sound (read) to me like her usual commonsense self either. So I think that maybe she had had a bad day - we all get those!
Lastly, to everyone who seems to think that to be "offended" by something or someone, is a weak or "snowflake" type of reaction to things, I couldn't disagree with you more. Without him even speaking, Putin offends me, and to a slightly lesser extent, so does Trump. The "parties" that Boris and his cronies held during Lockdown also offend me greatly. But I don't expect everyone to be offended by the same things, and for someone else to tell anyone else what they must be, or not be, offended by, is in my mind, just ignorant.
The OP did not come on here and tell everyone that they must be offended* *by that advert, she was just asking - as people on Mumsnet are want to do - whether other people agreed with her thoughts that M&S have made an error in showing that particular advert. I agree with her that it is a very inappropriate song, and whether they realise it or not (and they really should do their homework on these things) it refers to such an awful occassion, that they should have just not used it to encourage people to buy their merchandise.
Obviously, no-one has to agree with me, but I think that a persons method, and/or reasons for disagreeing, say much more about them than they might wish.