This plant is prunella, AKA self-heal. I found it growing at the side of a lane and didn't know what it was, but Bing 'Name that plant' came to the rescue.
Apparently the name means 'quinsy" a throat infection, and is derived from brunella, which is a diminuitive of brunus = brown. The plant was a traditional remedy for quinsy.
It's said to be edible and you can add it to salads but I'm not keen.