I have just been reading the link I put and the bit about what to put in a will is in the donation form and they have a large number of volunteers all over the country.
I reckon one would need to email them if you wanted to leave them a legacy and for them to take your cats on and of course someone else would have to know to contact them quickly but it would take the stress away if you also became terminally ill and also will try and get you into an animals froendly accomodation if you can no longer look after yourself.
Its sad they do not let people out there no more about them and their website has very limited information on.
There are a couple of good jobs going too!
Their patrons? are all very well known and apparently they have booklet they will send you.
I think its well worth looking into