White maggot les than 1cm
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More than likely the caterpillar of some kind of moth or sawfly species. Unfortunately the details on the shot are not clear enough for me to be able to hazard a guess as to which species.
I agree with Vinny, from the colour and the looping movement this is probably a moth caterpillar. Have a look at the tree canopy around the same spot and you will probably find many more eating leaves.
University of Edinburgh and Biodiversity Observatory (OU)
Lat/Lng: 51.3, -0.4
OS grid ref: TQ1263