Since you're in the UK I'd follow the example of Chris/Crystal Dale who died earlier this year of cancer. He was clearly a great character and there's a wonderful obituary of him at:
<http://www.telegraph.co.uk/news/obituaries/sport-obituaries/8346303/Chris-Dale.html>
which is well worth the read, but the important part as far as this conversation is concerned is:
Friends joked that they did not want to meet the woman he bought the clothes from. In fact, they were purchased openly, which led to one unfortunate incident in a Scottish branch of Asda. Thrown out for acting suspiciously, Dale complained in writing, citing the sexual discrimination act, and received a grovelling apology and a substantial voucher. It was with no little pleasure that he returned to the store to try on some clothes, soliciting the help of the manager who had evicted him.
Anybody given similar problems by a shop salesperson should follow Chris' example (the writing to management part, not the crazy mountaineering thing).