Age of consent in UK is 16, but you're asking about sexting which is different (18 min). Boyfriend is older than 18, your daughter is under 18. You said no images have been exchanged. The nature of conversation is important. If they are talking about sexual acts they'd perform on each other (not kissing, but like sexual you know what I mean), then that would be illegal. However, I'm not sure how seriously the authorities would regard this situation.
I don't see what the problem is though as no images have been exchanged.