The bolt head is removable, just unscrew it. The Brits provided various length bolt heads to correct for headspace issues as they were aware of the gun's propensity to "loosen up".
This is a widely-believed myth, at least in the US. The whole story about interchangeable boltheads applies only to the later No 4, and it has nothing to do with "correcting" some alleged propensity to "loosen up". Instead it was a production engineering development to allow a saving of time and labour in the production process. The idea was that rather than having a skilled worker fit up the barrel, so as to get headspace into spec., you would simply screw the barrel into place and then use the interchangeable boltheads to ensure that headspace was in spec., which could be done by a comparatively unskilled assembler and in much less time. It was only meant for the manufacture of new rifles, and really has nothing to do, as I said, with any "propensity to loosen up".
