In my experience of at least 35 years of independent shopping in M&S - I don't go as regularly as I used to because there isn't one close by - I can confirm, I have very rarely been approached by staff offering help, and that's been when there was a visible disability issue i.e. me trying to wrangle a basket with one arm in a sling.
Sometimes that policy of not approaching customers is exactly what you need. If I were to go into a specialist lingerie shop I'd expect to be asked if I need help. M&S on the other hand is a good place for a young girl who may be embarrassed to even talk to her mum about boobs, and certainly doesn't want to have that conversation with a random stranger.
If I were working in the bra department it would not occur to me to go up to a random unaccompanied minor and strike up a conversation about what would look good on her. The chances would be good that she would be mortified. If she were to approach me that would be a different matter.
If I weren't working in the bra department it would not occur to me to go into the bra department and strike up conversations with random customers, especially if they were unaccompanied minors.
If I were male... well, fill in the blanks wims, this looks like classic male pattern behaviour. Even if we're very charitable and it's just a TIM massively misreading the situation, that's what it looks like.
If you see it through the lens of male pattern behaviour, glibly saying that the girl could just say "no thanks" is a bit like saying a teen boy accosted by a nonce in the public toilets can politely say "no thanks" and go about his business. You have to be able to see it from the perspective of the person who is approached. Which some of our PPs seem to have a problem with.
If I were M&S, I'd be worried about the frequency of TIM staff popping up in lingerie departments. How many stores did we identify on the other thread, 8 or 9 at least? That looks like a disaster waiting to happen.