Amethyst, if everyone donated just £3 per month, the charities would not be short of funds, that's for sure.
I've said what you've said before, about not having another cat, and sometimes it's very hard to risk yourself in such a wholehearted investment again.
It's not about "replacing" one cat with another though. That over-simplifies it.
It's about giving another cat an opportunity they wouldn't otherwise have.
I've never "replaced" any of my cats. I've mourned their loss, and I continue to do so, but I also welcome a new character, a different personality, a unique individual with a great deal to offer and to share with us, for however long or short that may be.
I hope maybe one day you'll be able to make a place in your life for another cat or cats of your own, but if that's never possible for you, you're more than welcome to share ours, I'm sure.