They used to have a retirement scheme where you paid for the horse's keep by covenant which allowed them to reclaim tax on the money. Looking at the website I can't see whether they offer that service now, but I suppose it is worth ringing them.
Even if they don't, they may know somewhere that does.
i personally am not looking to send a horse to a retirement centre but its something i hav always wanted to set up, i live in scotland and haven't heard of any either, i wonder how much use it would be to people?
my mum and i would love to set up a business sometime in the future, and i have always pictured havin a private yard where we could take on elderly horses just wanting to live out there days happily, gettin the best treatment