I'm the one who has been trying to get my partner to be on board with a joint account for a long time. We've just got one sorted.
Due to me being on maternity leave with our second baby, and having sacrificed my business for 2 years due to a 2yr old and 8 month old, when I return to work in march, even if I went full time (which I'm not), my earnings will be significant lower than pre maternity.
As such, we worked out all the family expenditure (bills, groceries, things for kids, nursery fees etc) and our income (mine maternity allowance and btl income minus personal pension, & his post pension pay), then paid in proportionally. Atm me 30%, him 70%. Makes way more sense that way. Tho, still (not that he thinks so) is biased towards him, as he has way more spare money than me due to working full time, whereas I do more childcare (even on days I'll work, it's only me that can do nursery drop off and pick up due to his hours. When I start earning again I'll pay a bit more in, when I'm earning ok again a bit more again.
Holidays one of us pays, the other transfers half the money.
I definitely think there should be a joint bank account when living together. But not necessarily full incomes