I would concur that there are no figures for glucose levels written in statute. At least I have not been able to find them during several previous on-line searches. The underlying principal of course is 'Fitness to Drive' and how it is assessed.
The Secretary of State's Medical Advisory Panel sets recommendations for group 1 drivers and requirements for group 2 drivers.
However, I think this misses an important point. When an insulin dependant diabetic applies for a license to drive, the second question asked (either on line, or on the printed form) is.....
"I agree to test my blood glucose at times relevant to driving. Times relevant to driving means no more than 2 hours before the start of your journey and every 2 hours whilst driving."
The applicant ticks either the YES or NO box. (I'm assuming that no person here has ticked NO and still managed to get a license?)
At the end of the process the applicant will sign a declaration that the information provided is correct and thereby enter into a legally binding contract with the DVLA. Whether there is law to govern blood glucose monitoring then becomes largely irrelevent.
My understanding then is that if after obtaining the license to drive, the applicant did not test glucose levels it would mean that the license was obtained under false pretences and would be invalidated.