Expiry is related to each individual account
I don't think that's universally correct, although it is in some circumstances.
If none of the members have status, it is correct.
If one or more members have a certain BAEC status, then activity on that account keeps the others alive. There is a little bit of uncertainty over what status that has to be, e.g. in the link off the Dashboard here:
https://www.flyertalk.com/forum/21028657-post1.html
In the Section "Keeping Avios Alive" is says it should be Silver or Gold. However, BA's own T&Cs on Household Account suggests Bronze is enough (emphasis below is mine)
18.1.18. Notwithstanding Clause 18.1.16, in the event a Household Account has a Gold, Silver or Bronze Tier Member who has earned or redeemed Avios points within the last 36 consecutive months then the Avios points balance of the other Household Account Members shall not be subject to expiry in accordance with Clause 14.4. In the event a Household Account ceases to have as a Household Account Member any such Gold, Silver or Bronze Tier Member then the provisions of Clause 18.1.16 shall apply.
From: https://www.britishairways.com/en-us...useholdaccount