You can only buy fireworks (including sparklers) from registered sellers for private use on these dates:
15 October to 10 November
26 to 31 December
3 days before Diwali and Chinese New Year
At other times you can only buy fireworks from licensed shops.
You can be fined an unlimited amount and imprisoned for up to 6 months for selling or using fireworks illegally. You could also get an on-the-spot fine of £90.