The last time I read about this all you had to do to designate your house/property as an explosive store was to contact the council and inform them and inform the local fire brigade so they knew what to expect if called out.
It may have changed but if you do want to classify your property as an explosive store it might be worth checking with the council or fire brigade.
Primers and the powder in loaded ammunition now count towards the maximum weight of stored explosive. I have the figures somewhere I'll see if I can find them.