they always were incinerated, although with the scrapping of the OTMS, cattle over thrity months old can now enter the food chain. so if a cow at the end of her milking life is still healthy then she can enter the food chain. most are usually suffering from chronic mastitis or other problems though, so get incinerated.
pet food is of a higher grade than human food in this country, so nothing that cant go in the human food chain may enter pet food manufacturing.
Theortically they can now the OTMS has been abolished (over thirty month scheme). During BSE we were not allowed to have anything enter the food chain over 30 months. Years ago old dairy cows were known as the "MacDonalds Special". I'm pretty sure they wouldn't be passed for animal consumption these days and yet were allowed in our burgers, go figure!
We do have barren cows that go off which are completely sound and do not have mastitis (we pride ourselves on our dairy hygiene and animal husbandry which helps prevents mastitis). However we find lameness a big issue (from foul of the foot to ligament damage in the hips etc) with the older cows and since cows have to be 100% healthy now to walk on for slaughter it rarely happens. They used to be allowed to hobble (when I say hobble I mean limp, not on three legs) on and off the waggon, not any more.