Unidentified insect seen in southern UK, near North Downs
Though it might resemble a woolly aphid (https://dengarden.com/gardening/Woolly-Aphids-Whats-that-Fluffy-White-Stuff-on-my-Tree). I'm more certain that it is a mealy bug instead (https://puffycarrot.com/little-fuzzy-white-bugs/). the fact that it is not on a plant is what confuses me since both these species are usually living on plants which is where they grow.
here is some images i found that look almost identical to the photo you have and give other angles to the species, you can compare it with the images in the other links:
https://www.flickr.com/photos/unique127/2539022716/
https://www.ecosia.org/images?q=white+furry+insect#id=DDEE9FF71839419EF585778C7DA8F1E1545E98D1