I guess it fundamentally comes down to what you are looking for in a grade. To you want to identify the 'best' 5% of maths students in a given year or identify everybody who is at least 'this' good at maths.
I think grading on a curve means that student won't study in a collaborative manners: they don't discuss solution, sharing knowledge, etc... Which is the opposite of what we want.
But a nationwide curve brings back the incentive for collaboration - your group of friends or school will still push itself up compared to other schools.
In a country as large as the UK, a curve wouldn't really make me stop collaboration.
The SAT IIs were curved in the US and I saw no such effect.