Oh dear god, the A421 is a nightmare east of MK. With this at one end and Black Cat at the other...all seems lost. There is hope, however, and let's pray the Highways Agency are listening on this one.
It's called...the A422. It's dualled through Milton Keynes, is rarely busy, and runs through a grand total of one village (Chicheley) between MK and the A428. Add a Junction 14A here, and problems with the Ridgemont Interchange *should* lessen.