Again, I'm not disputing that there is method in why the UK is doing this and I won't pretend to be a doctor, a public health official, a data scientist or whatever. I'll leave calculations on whether you are 50% protected, or 60% or, 80% to those who have studied the subject.
All I'm merely saying there is potential for issues in recognising the validity of one's vaccine due to the way it's administered. Until the fine folks at the WHO, or the EMA etc rule that the 12 week interval is OK there is potential for countries not to recognise the vaccine's validity as in the case of Iceland.