Unless you apply to the HSE for a licence to acquire and keep smokeless powder it is illegal to do so in any quantity, however their are exemptions (as below) for a quantity up to 15kg but that is total NEQ (net explosive quantity) so includes powder in loaded ammunition. Above 15kg a licence is required.
Up to 2,000kg a police licence is required (acting on behalf of the HSE) above 2,000kg HSE issue the licence.
Each police force has an Explosives Liaison officer, normally within the firearms licensing team, most likely also a FEO.
To comply with Heath and Safety Explosive Regulations 2014 (2014 No.1638) to acquire smokeless powder the person must be either:-
(1) hold a firearms certificate granted under section 27 of that Act; or
(2) hold a shotgun certificate granted under section 28 of that Act; or
(3) hold a permit granted under section 17 of the Firearms (Amendment) Act 1988 or
(4) registered as a firearms dealer under section 33 of the Firearms Act 1968
Black powder needs a licence to acquire and keep regardless, issued free to certificate holders.