02 refunds "policy"

I went into my local O2 store to buy a new phone charger for my partner. I am not an 02 customer (except in the sense that I went into one of their shops). About half an hour later, I found more or less the same thing for half the price in a local luggage shop, so took the charger and the receipt back to O2. I was asked for all sorts of personal information before they would give me a refund - my name, address, postcode, home and mobile telephone numbers and date of birth. They said that this was "policy". I told them it was my "policy" not to give such information. They got very, very sticky about this and continued to press me for my personal details. We were heading for a major stand-off with me refusing to give my details and them refusing to refund my money if I didnt give them. I asked several times why they needed my details given that I was not one of their mobile customers and, given their attitude, not likely to become one or indeed ever return to one of their shops. They went no further than repeating the word "policy".

I made a formal complaint to O2 about the treatment I had received and their reply included the sentence "It is common retail practice to obtain personal details from customers returning items for refund".

I have never come across this "practice" before and found it extremely intrusive (and a source of potential junk mail/irritating sales calls). Is their "policy" enforceable by law and should I have stood my ground as regards my personal details? Where would I have stood legally if I had refused to give them?