Possibly. You should check with your local trading standards department. The information you will need is the hazard type (Usually 4), the Net Explosive Quantity (this can be found on the side of the shipping box as NEQ or NEC) of the pyrotechnics you will be storing. You may also need to know the UN Numbers for pyrotechnics; these are 0432 for 1.4S and 0431 for 1.4G. The laws regarding the storage of explosives changed in 2005, you should check the new MSER regulations to ensure you comply. Below is a brief "in a nutshell" version of how MSER affects pyrotechnic users.
Please note that these weights only apply to the explosive content and not the gross weight. Explosive weights can be found on the delivery box (NEC or NEQ mark).