I think also when you tell your friend that you just can't afford it, you need to say nicely that her latest revelation about having to spend so much was a surprise, that you have both been making assumptions at cross purposes due to your own traditional expectations - that obviously you weren't expecting all these bridesmaids expenses as the wedding is a UK based wedding and over here it's the bride that pays for the bridesmaid's expenses such as dress, hair and make up. And that you understand that the tradition is different where she comes from, which is fine, when there are longstanding traditions that seem such a basic part of things that nobody questions them, you take it for granted that it's the same the world over and it doesn't occur to you to ask elsewhere. But given that it is different over here, you've not had any notice to give you time to save up (albeit you recognise that she might have assumed that by asking you she was giving you that notice, she has to recognise that because you are used to UK traditions, that wasn't the case), that you are on maternity leave so short on money anyway, but the reality is that (much as you'd love to be a millionaire and be able to afford everything without thinking about it, you're not) and that the maximum you can afford to contribute towards your dress is £xx and that you'll do your hair and make up yourself.
Hopefully one of the other bridesmaids will be good at hair so can help and if the bride doesn't want to pay anything more then she will have to prioritise - your hair/makeup vs you not being a bridesmaid or not having something else...
Unfortunately she will probably have budgeted for the wedding on her mental model of weddings - ie that the bridesmaids pay for their own stuff (and she's probably paid out for stuff if she was a bridesmaid at previous weddings) so won't have lots of budget to suddenly fund lots of bridesmaids stuff.
I suspect this is another reason why in the US etc it is much more popular to have lots of bridesmaids - if you don't have to pay for them whereas having to pay for them tends to concentrate the mind and wallet 
Just because she expects you to pay because that is her custom doesn't mean that it is unreasonable to to explain to her why you weren't expecting to pay and that you can't afford much!