Nationalism + sports = 
Let's scratch the Olympics, World Cup and so on and so on.
Why not! Great Britain/England/Wales/Scotland/NI are crap at all of them! 
Lol - COMPLETELY derailing my own thread: How come does England, Wales, Scotland and North Ireland compete as Great Britain during the Olympics and Formula 1, but in soccer they are allowed to qualify for the tournament with 4 different teams? It's an unfair advantage!
Historic and yes, no fair.
You'd have to clarify which you mean is the unfair advantage. They way you've written it, you're saying that having four different teams is the unfair advantage which the complete opposite of the truth. The advantage would be in pooling resources and having the option to choose the best of all four nations in one team, so if - say it was a few years ago (because even though he's a bit older and actually is in the Olympic squad, he's obviously not as good as he was when he was 30) then Ryan Giggs, who is Welsh, would be able to have been picked by a GB football team, when he was consigned to play for Wales for his international career. Could you have imagined a GB team in the world cup with George Best in it? Or Kenny Dalglish?
It's a disadvantage to have four separate teams, not an advantage.
It's not purely historic either, it's related to professional football associations, and there is a different PFA in England, Scotland, Wales and Northern Ireland.
For athletics it's different. GB competes as one team in Olympic and World events (an advantage), but separately in Commonwealth events where they are competing against each other.
Favourite racing series: F1, Indycar, NASCAR, GP2, F3, Formula E, Trophee Andros, DTM, WTCC, BTCC, World Endurance... etc. etc.