You have my every sympathy, middleagespread, never underestimate the way you/we feel about them, they are part of our lives and we miss them terribly when we lose them, even if we know that we are making the right decision when it comes to that awful moment.
I knew straight away that I wouldn't be happy without a pet (cats, in this case, although I used to have both, as in cats and dogs) and although no cats will ever replace my last three (we moved into this house with the 3 of them, all now resting in the garden
) I now have 2 lovely boys from Cat's Protection that bring me so much pleasure.
I think that both the RSPCA and the Dogs Trust have schemes for what they call their "golden oldies" (may not be quite the right thing, but I'm sure that if you Google it you will find something!) They are keen to look for homes for older dogs that many people overlook because they want puppies or younger dogs. I think with this scheme they will pay (or make some allowance for) vet bills.
Give it some thought, it might be the ideal thing for you, but of course in the end it is only you that will know.