What is For Britain!?
1.
Used sarcastically/mokingly: When someone does something so well or often that they could represent Britain in the Olympics (or other national event) if the activity that they are good at were an Olympic sport or similar.
"Kate shops for Britain!", "I know she spent a whole day trying on shoes!"
"Chris sleeps for Britain!", "I know he didn't get up til 3 in the afternoon."
See