I'm not sure what is exactly going on here, but Ants and many other invertabrates have special relations. This could be a caterpillar of some of the UK butterflies.

I'm going to try to get ahold of someone that can help me figure it out.

This wasnt the only caterpillar being moved along the cliff edges amonst the thrift, probably being taken back to the colony for night, then will be brought back to the food source for the day. The ants harvest honeydew from them.

You can see the Ant has a careful hold on the caterpillar in its meaty parts, he was carefully carrying it along, the caterpillar seems a bit curled as they do as a defence when being handled and has a 'poo' so has been eating and seems to me alive and well.