The one I know about is in the Milton Keynes area. Briefly they keep about 250 horses in herds of 50'ish. They live out until about November when they are brought into big barns for the winter. For taking blood, farrier visits, etc they are always brought in as a herd.
If you are thinking of sending a horse there, they have to be under 16 yrs and over 16 hands. You sign the horse over to them with no further contact.
The manager is David Toomes - 07801 639510 - and is a lovely chap. They usually take horses in twice a year.
Sorry you've had to start thinking of this............good luck