Elendil has covered the idiomatic aspects of "football voice", but the pronunciation characteristics are somewhat different. I'm not from the UK, but my understanding runs as follows: both Moss and Jenn speak with a typical educated Londoner accent, which falls somewhere between the lower-class dialects and the upper-class RP ("Received Pronunciation"). This is the closest thing there is to a neutral accent in Britain. However, when Moss switches into "football voice" talking to the (presumably working-class) postman, he affects a style of pronunciation that's more associated with lower-class Londoners, which is how the stereotypical football fan speaks. The football voice is thus a combination of football-specific idioms and a particular dialectical pronunciation.
British sportscasters are probably not as working-class as their listeners, so I suspect that if you hear sportscasters talking in this way, it's partly an affectation.