I found a lot of male Whatcotts living in two districts of Gloucestershire - Chipping Campden and Sub-Edge. Both places are not far from Honeybourne. I found them in the 1901 census - using intelligient person search but I did not pay to view the households - www.1901censu.nationalarchives.gov.uk - so for them to have been your grandmother's adoptive parents in 1920 - we would have to add 19 years to the ages given in the census.
There are not many candidates - Thomas Whatcott age 13 Gardener's Assistant seems the most likely as he would have been 32 in 1920 - young enough to take on a family.
You may have to subscribe to ancestry.co.uk to find out who he married. Can you get your grandparent's marriage certificate and check who their witnesses/ parents were on that?
Good luck
Linda