You should know that - you must be at least 16 years old before you can buy any fireworks. There will be a warning sign in the shop telling you this. If you have any or use them you are committing a criminal offence. Otherwise, you are breaking the law. Otherwise, both you and the shopkeeper are committing an offence.
Sales are now in boxed sets.
The legal penalties for breaking the law are severe. You could be fined up to 5,000 and have your fireworks confiscated. Such premises are required by law to display their registration certificate. Check that they have one before you buy. Look for the British Standard description on the firework or box which means that this type of firework has been tested to meet certain safety standards.