As far as I can work out: if you have a couple of hives in the garden, BDi will cover them whatever any other member of your association does. If, on the other hand, you move one of those to an association apiary, there's a good chance one of the other occupants has a split more than they estimated, is late with subscription, unclear on ownership or otherwise outside the restrictions so that moved hive is probably not covered. Your hive left at home is still covered and you have acted in good faith.
There are a couple of features of the system that I'm pretty sure many members are not aware of. One is that hives on any association apiary are unlikely to be covered. The other follows on from that in that if you have two hives at home and two in the apiary, you still have to pay for the higher band of 5 even though you know the two at the apiary are not covered. You cannot opt to pay the basic rate for the hives at home and leave others outside the policy.
I'm not aware of any other insurance company that insists you pay premiums on items that you and they both know will not be covered.