According to the Firework Regs 2004 there doesn't appear to be a requirement to have storage or insurance to be supplied Cat4 fireworks.
There is however a specific offence to be in possession of Cat4 if you are not a Professional Display Company or employed by one. There are a list of exceptions for Council and Armed Forces etc.
So my understanding is even if you got a company to deliver to the site and you had single event insurance, if you are not a Professional or employed by one you are committing an offence just by being in possession of Cat4....
Might of course be totally wrong....
As a Professional Display Company you don't need a 1.1g store to be supplied or be in possession of 1.1g if it's direct to site for use, so do you actually need any store??? But then can you be a Pro Company without one???