It’s not always easy to identify caterpillars but this has some distinctive markings and was in very good light which helps, even though it was moving at speed!
It’s a final instar larva of the Buff Ermine moth.
Insight into Buff Ermine (Spilosoma luteum): the larvae feed in autumn on a variety of plants. In its final instar it measures approximately 40mm, has a dark grey body, paler grey dorsal lines and a light brown head.
It will overwinter as a cocooned pupa.