About 20mm can't help with food plant as it was going for a walk on the beach
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Four-dotted Footman would be quite a surprise for the locality. If it were that it would have a shiny black head, but the head in this photo appears to be dark reddish. As to the size, 20mm is reasonable for a larva just out of hibernation, but bear in mind that parasitised larvae often develop more slowly , and so can be relatively undersized.
Lat/Lng: 50.718, -1.802
OS grid ref: SZ140909