See page 100 HSE document below states it should be stored not it shall or must be stored, go back to basc and ask the question again because I am surprised by the advice BASC has so far given you. Also ask them and the FEO to put it all in writing. As has been said lots of shops would be wrong to display openly smokeless powder on their shelves.
Should is used to mean a recommendation only
1 Regulation 27(3)(b), (c) and (d) disapplies separation distance requirements for keeping small quantities of shooters’ powder, water-based explosives and detonating cord. The following sections describe the conditions that should be met to qualify for the disapplication. These conditions apply at all premises including domestic premises
Shooters’ powder
2 Shooters’ powder includes both black powder and smokeless powder.
3 The powder should be kept in containers with no more than 1 kg of powder
per container. The containers should be constructed in such a way that, in the event of a fire they do not provide additional containment that will either increase the explosive force of any deflagration or cause smokeless powder to detonate. Normally plastic/polythene or paper/cloth containers should be used. Metal containers with a screw cap or a push-in lid must not be used.
4 Although shooters’ powders are generally not very sensitive to ignition by electrostatic discharge, homeloaders or others who decant the contents of plastic containers should take care to reduce the risk of static electricity build-up. Advice on precautions may be sought from the manufacturer.
5 The containers of powder should be kept in a box constructed of plywood with a minimum thickness of 18 mm and a maximum thickness of 24 mm.
6 Where the box holds more than one container, each individual container should be separated by a 6 mm wooden partition that is securely fixed to the outer walls of the box. Each compartment should allow 30% additional height between the top of the container and the inside of the lid.
7 There should be a good seal to prevent a flame path between the lid of the box and the interior. An intumescent strip should be fitted around the edges of the lid to give a good seal or alternatively, a stepped lid or internal second lid should be fitted.
8 The box should be constructed so that there is no exposed metal on the inside. Internal nuts should be covered by a glued wooden liner not less than 6 mm.
9 Figure 1 shows a box that would meet the requirements set out in the previous paragraph.
See
personally I would keep black powder in a wooden box as detailed above.