BAEC membership doesn't normally expire as such.

There are two types of thing that do expire.

First, your membership year will end on the same date each year. It's always the 8th of a month, so it sounds like 8 December is simply your membership year end. The membership year is used for the purposes of determining your membership level or status, which is fundamentally based on the number of Tier Points earned during a membership year. But the end of the membership year simply resets that counter to zero. It doesn't terminate your membership or cancel any Avios that you've earned.

Second, Avios themselves will eventually expire, but on a timetable not linked to your membership year. This normally happens three years after the date of the last substantive activity (earning or redeeming) on your account. (Transferring Avios between the Avios-earning schemes doesn't count.) So if you redeem some Avios now for flights, that will reset the clock for this purpose. Whatever Avios are left in your account after your redemption should remain alive for at least a further three years from the date of that redemption.