I'm going to go back and change that post!
Yes, a legal responsibility. Now, for this next bit, I'll say now, feel free to google and nitpick, as I'm giving the legal definition of an 'accident' ad hoc from memory here, so doubt I'll get it word for word as it is in the Road Traffic Act...
'If due to the presence of a motor vehicle on the road, an accident occurs whereby ... (secondary bit is about what constitutes 'an accident')'
So yes, if a motor vehicles presence on the road has any part in the accident, then they are considered as a motor vehicle involved in the accident, and thus have responsibility to exchange details. Legally. Technically.
So yes, Bicycle - the bobbies that dealt with you 'technically' got it wrong. You should have exchanged details, however applying a bit of common sense, if neither of the other drivers thought you were to blame, if the officers didn't think you were really to blame (and for the record, IMHO you weren't!), then they may have actively decided it simply wasn't worth it - besides, they had your details later if needed.