Asking for their number might well get you a "right you're nicked you cheeky fucker" - you will probably be taken for a public order offensive and even resisting arrest/assaulting a police officer.
Really, I did it every time I got stopped during my time as a courier, which was frequently, given the way I rode, it worked every one of the 40 or so times I had a run in with them....only ever got one ticket.
Making a note of their number shows that you know your rights and you're not going to be a pushover, they're less likey to falsely arrest you if they think you might genuinely follw it up.....
Really, I did it every time I got stopped during my time as a courier, which was frequently, given the way I rode, it worked every one of the 40 or so times I had a run in with them....only ever got one ticket.
Making a note of their number shows that you know your rights and you're not going to be a pushover, they're less likey to falsely arrest you if they think you might genuinely follw it up.....