Rugby union is only really the number one sport in a very limited part of mostly rural west England. It's obviously not even the main code north of Nottingham. In most of the country it's primarily a middle/upper-middle class sport but even then has to compete with Olympic sports the UK does well at nowadays alongside football or cricket.
Other than the front 5, the physical differences between rugby players and other sports folk is gym work and diet.
Other than the front 5, the physical differences between rugby players and other sports folk is gym work and diet.