As far as I am aware, this is beyond reasonable requirements or have you had a premises survey and been so advised - from the letter itseems a policy change and in contradiction of the Firearms Security Handbook?
Quote
2.1 For these purposes, a “substantial” number of firearms should be considered with regard to the type of firearms, their potential danger if misused and their likely attractiveness to criminals. At the lower end the number might vary between six and ten, depending on the type of firearm concerned, whilst anything over ten would rarely be lower than level 2. It must be stressed, however, that it is not enough to base an assessment on the number of firearms alone – all factors mentioned above and in paragraph 2.20 should be taken into account. Sound moderators, spare barrels, spare cylinders and component parts should not be considered as part of the total.
End quote
A humane condition would make that level 2 without necessarily reaching 6 firearms.
The security handbook is quoted on here and you should perhaps send them a copy and explain why you find their requirements unreasonable.
You could always try BASC and hope they were not one of the orgs consulted, who sold you out. Maybe they will take your case, then again maybe not and since your only hope of insurance is your household policy - I'd read that after having read the above and decided.
Clearly burglary isnt as important as you having to pay to prevent the police from failing in their duty to protect the public - from burglars.