Crawling across pavement under large trees
No interactions present.
Having looked at your photo I am inclined to think that this may be the caterpillar of the Eyed Hawkmoth (Smerinthus ocellata).
The Poplar Hawkmoth larvae and the Eyed Hawkmoth are very similar in appearance but the tail of the Poplar Hawkmoth larvae tends to be yellow whereas your photo shows a larvae with a glaucus colour to the tail. This is generally the coloration for Eyed Hawkmoth.
I'm sure there are others on here who will help too. Don't just take my word for it :-)
All the best
Pete Burton (Cumbria)
Lat/Lng: 52.6, 1.2
OS grid ref: TG2006