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If you found it at this time of year then it's almost certainly Angle Shades, they tend to feed in spells of mild weather.
Of course a side-view is often critical for 100% caterpillar ID.
Lat/Lng: 50.4079, -4.105
OS grid ref: SX505585
There is ivy (Hedera helix)growing on the rotten fencepost on which this insect was photographed.