Its vespa velutina that's been spotted in Gloucestershire.
See the National Bee Unit's post here
and info in the species here
They aren't particularly large and I think the clearest identifier are the 'yellow socks' it sports (that and the yellow band on the 4th segment of the abdomen, but it's size varies and sometimes is more difficult to make out).
This is a particularly bad time for it to have been spotted because about now, any nest will be incubating hundreds of queens which will mate with drones and seek out locations for new nests. If they aren't found and destroyed very soon, the species will only be controlled by the cold winters, which are getting warmer..