Is this a valid reason to be stopped by the police?

So, I was pulled over in my boyfriends car this afternoon on the motorway (in UK). The police officer asked whos car it was then asked me to go and sit in the back of his police car as he "wanted to talk to me about the car". He asked me "when you came on at Junction 31 did you cross the double white lines in the middle of the road?" to which I answered no - because I hadnt!! He then said he never saw me do it, he was just asking! He then went on to tell me that it is illegal to cross the white lines and next time I was pulled for such reason I would get 3 points. I, by this point, was confused! He then went on to check that I was insured on the car (I was as I can drive any car under my name). He had already checked and knew that I wasnt covered under my boyfriends insurance. He then told me that if I hadnt been insured then they would have took the car of me. I am now getting more confused. Why is he telling me this when everything is ok? So I then asked him if he pulled me because of the "white line" incident. He said no and that he just wanted to check I was an insured driver, however he went on to tell me then that I was being given a verbal warning and I was lucky to not get 3 points on my licence for driving over white lines, I replied saying I hadnt! He then let me go. Sorry for waffling but it has really annoyed me - arent there murderers and idiot speeders around that he would be better stopping? Maybe I should start driving around with my bum out of the window, he would have had a better reason!! Is this legal for him to do this? And is a verbal warning on a record anywhere?