I don't think for a minute RR wanted to see Sabrina Carpenter, anymore than KS wanted to see Taylor Swift, but when offered impossible-to-get tickets I reckon she got them for her child. If they were £600 they're going to be VIP tickets at the front, maybe even meet and greet ones.
Instead of the having to declare it rule, why not just a straightforward No? ie you don't take anything, not a ticket, not a sports trip, not a free lunch. In politics there's not a single thing that comes without strings attached, even if it's just the ear of a minister for 5 minutes. And no 'guests', business or otherwise, in the subsidised restaurant at the Lords or Commons either, unless it's close family or a school trip.