Oh, you must back up your sensible bridesmaid ally - I think you do need to find a way to speak to her about this face to face, as texts can be taken the wrong way.
I think someone suggested you meet her for a coffee, just the two of you - that could work, you won't even need to think 'how am I going to bring this up?' as you said that all she does is bang on about the wedding. I think you'll just need to be very blunt and honest, it doesn't sound like she'll respond to subtle hints that she's asking too much of you.
You've already made a list of the breakdown of costs - you could show her that and explain why you needed to cancel the hotel, and that you just can't pay any more! You're still getting her a wedding gift (although I am resentful on your behalf that you would have to)..."[other Bridesmaid] thinks you are expecting a gift hamper from us the night before the wedding....you've not really asked for that, have you?"
It matters NOT ONE BIT if she is planning on getting you all goody bags. It is quite common to give bridesmaids a small gift as a gesture of appreciation, and frankly it is the least she can do after making you all fork out for your own (expensive) dresses, shoes that you will feel uncomfortable in and never wear again, makeup, hair, nails...I for would bet that whatever she is getting you, she is expecting much more valuable presents in return, hence the £45 per person.
(FWIW, if I was getting married and had to ask bridesmaids to cover their own costs, I would let them pick their own dresses and emphasize that I would not expect a wedding present from them, nor was there an obligation to take part. But that's just me).
Nobody can accuse you of being tight, not with your money or with your time. You have been so patient to listen to all this shite for years. But you must get this out in the open sooner rather than later, and if she sacks you then so be it.
(The waiting for gasps thing is hilarious, btw. Please keep us updated on the whole situation!)