HSE has published guidance for retailers on storing and selling fireworks safely:
You can keep up to 50 kg (net) of Hazard Type 4 fireworks for no longer than 21 days without the need for a licence, provided that these are not for sale or for use at work. This is intended to help private individuals and voluntary organisations that wish to organise a larger firework display. (Hazard Type 4 fireworks are the types of less powerful fireworks generally sold at retailers).
If you wish to use the fireworks for a commercial display or for some other work activity, you can keep up to 100kg of hazard type 3 fireworks for up to 5 consecutive days without the need for a licence, provided they are kept in their place of intended use.
If you wish to store a larger quantity of fireworks, or more powerful fireworks then you will need a licence from your local licensing authority.
If you wish to buy more than 50 kg of fireworks you will need to produce your licence.
If you are in doubt please check with your supplier or with your local licensing authority. Please note that the requirements of the regulations to store the fireworks safely will continue to apply.
The Health and Safety at Work Act 1974 and subsidiary legislation covers firework displays which involve a work activity (ie where at least one person will be involved on a professional basis). This legislation places duties in respect of the health and safety of everyone involved in arranging and giving the display, the spectators and other people near the display site.