TOT
How many times have you visited Caernarvon before? It's almost worth sending the £25 to the CEO of the operating company with a note saying - herewith payment for the fine re hi-vis. I have visited Caernarvon XXX times in the past XXX years and will now no longer do so hence you have lost XXX in landing fees.
Otherwise just ignore it or tell them you were wearing it, just not as the top layer of clothing.