How long can it take to get to the National Bowl (Milton Keynes) ?

I'm driving up to the Bowl on Sunday to see the Foo Fighters! It's about a 6 hour journey to MK usually from where I am but I'm guessing it could be quite busy. Also I'd like to get there as early as possible. Gates open at 2pm. Does the traffic get stupid around MK for these events and can anyone suggest how long I should allow?