No there's no problem with other metal containers as long as they're suitable secure, weatherproof and fireproof. People use ISO containers presumably because they're the most readily available such article, though with their current price being quite high if you know someone who's handy at welding you might be able to make something from scratch a bit cheaper.
Unlike the storage of high explosives for which the method of store construction is prescribed by the HSE (HSE Circular to Chief Officers of Police 1/2005) the storage of fireworks of any category does not have such a document. What is prescribed however are the safety requirements such as lightning protection and general storage housekeeping rules established by the Approved Code of Practice to MSER 2005. There is a general security provision still found in one of the remaining sections (S23) of the 1875 Explosives Act which says that the occupier of any place where explosives are stored shall take all due precautions to prevent unauthorised access to the explosives therein. Basically, keep your store secure!
So you may construct your store of any suitable material, make it compliant internally with MSER (ACOP), secure it to comply with S23 of EA 1875, site it to comply with the separation distances of MSER and you should have a happly licensing officer!
Edited by exat808, 31 August 2010 - 02:06 PM.