I saw this insect today on the Dollypers Hill nature reserve, Surrey.
It was on chalk grassland mainly feeding on wild oregano/marjoram flowers.
Not the best picture - I know, as only its abdomen / wings are visible. It has a quite rounded / bulbous abdomen compared to common wasps. It wasn't furry so I thought it was more like a wasp than a bee.
My first thought while out walking was a hornet - however when I searched for pictures of hornets - it didn't seem to look like that either.