We wore the Mk1 bonedome and inner because that was the design issued at the time; the Mk2 was issued to the fast jet world only. The Mk 3 came out, was issued to the fast jets and the Mk2 was made available to Vulcan crews. It had nothing to do with headspace in the cockpit. I wore both at various times and never had a problem.
The question of whether the cockpit was designed for one or two pilots has been the subject of much rumour an speculation over the years. I don't know the definitive answer, but I suspect that it was a 2-man design all long. Perhaps people got confused because the Avro 707 series was designed to fly with a single pilot - they were design testing platforms, since no-one had practical experience of delta aerodynamics.
And the one that broke up at Syerston was a Vulcan - see the photo above.