As I understand it, the chemical compounds in flares degrade with time. Hence manufacturers put a credible replacement date on them so they are not liable for flares that fail to ignite or, worse, burn incorrectly after that time. Having been professionally involved with pyrotechnics and explosives, I tend towards safety rather than economy.
No intent to start a war with this!!
Print off this leaflet and take it to them. Halfway down on the right of the second page it tells you to take them to the police. Newport IW have always taken them with no problem.