they want 18-19s tax return which was very low for me as I was employed half the year
As you say it varies from bank to bank but I read on at least couple (including HSBC I think?) that if started being self employed part way through the tax year 2018-2019 or calendar year 2019, they will take the portion that you did work and pro-rata it to get an annual amount, which they will then x25% to arrive at the figure you can borrow.
It's a loan at the end of the day that you/the govt will pay interest on so I suppose it's in their interest to approve you for borrowing as much as possible. This is what I was hoping they would do with the SEISS grant too as I started towards the end of 2018/2019 tax year, but unfortunately they haven't, I spose bcs it's a grant and not a loan.
A couple of the others also allow you to open a feeder account as mentioned if you are a personal customer with them.