Ok, I agree with Kate (ha ha, not often that happens ).
You interview THEM first, find the ones you like, narrow it down to about 2 or three (you may only have to stop at 2 or 3 before you find the one you like). When you are meeting them you tell them about your current pregnancy, they'll ask you about previous ones. You tell them later in the meeting that you would like to talk about a payment plan because you won't have the funds before the birth, and you would like to discuss flexibility in the fee. They all have set rates, and I know that many of them are willing to work around hardship (my poor MW's waited almost 6 months to be paid off, and that's considering we hadn't worked out a payment plan, we just got stuck with DP's job change and pay cut and waiting for house sale funds to come through) to get women the birth they deserve.
Yes, it isn't fair that we use our baby bonus on the birth, when one could have all the bells and whistles of a medicalised birth for free...and this is where we were at after DS was born, but when I knew I was pregnant with DD I didn't care about the politics anymore and knew I'd give up my baby bonus for the birth I needed to have (not that DS' was regrettable, it just wasn't at home!).
So, hold off the letter, just interview midwives (Midwives Naturally have a Meet the Midwives night one Tuesday a month where you get to talk to all of them in one night and choose your primary and secondary - their policy of two middies pushes their rates up more than others, but I actually like the idea of a secondary one) and work from there. All midwives I know just want HB to be accessible to everyone. Without Baby Bonus it would get trickier to secure payment from people, but whilst we have it there is a way to afford HB
Good luck