Hi,
I am currently fostering a mother and her litter of kittens for the Cats Protection League and soon will be working with them to home them.
I do hope you won't be offended by what I have to say, these are my thoughts and not nescessarily shared by others:
I understand that it can be hard to find £50 in one go, however cat charities ask for this for more than one reason, the first which has already been explained is that the "donation" will usually cover the cost of first vaccinations and neutering (You get a voucher for this if the kitten isn't old enough to have the procedure at the time of homing), the cost of this alone would come to at least £50 at most vets anyway with no account taken for the costs of caring for the queen and raising the kittens.
Another reason not touched on is to ensure that the person taking on the kitten/s has sufficient financial means to fund care should the kitten become ill. If you can't afford to pay £50 now, what if you have unforseen vets bills, or can you afford pet insurance etc?
This is a quote from a book refering to breeding of rabbits, but I feel it is appliciable to any animal "Protect your babies with a respectable price since it is human nature not to care for something obtained too easily"