I cant confirm this but my guess and assumption is that anything other than account sharing between everyone, you will have to pay for a box for each account and sub for each account. I have never played an MMO that any sort of exception based on your household numbers. If you share an account not everyone can be online at the same time.
Playing games is a luxury aswell so I see no need to have this special rule at all aswell no offense but its not a basic good you need, if you cant afford the 300-400 pounds a year then maybe you should reconsider getting everyone playing the game. Or maybe got for a Free to Play game where you can limit the money "consumption" on all the accounts.