Assuming the Telegraph reporting is accurate, is failing to report falling asleep at the controls normally a dismissible offence?
It can be, but it would normally involve some other steps.
If a colleague pointed out that you were up playing FIFA until 4am before your 7am shift on a regular basis, and you knew you were regularly falling asleep or having fatigue issues, you could perhaps expect to jump to dismissal, especially if you're just argumentative about it.
On the other end of the spectrum, if you've got 20 years clean record, then in your 50s seem to develop a pattern of fatigue, it could point to a medical problem. My dad is a driver and was taken off a couple of times while they got his sleep apnea under control, a condition he developed after 40 years as a driver. He got a CPAP machine and made some lifestyle changes and now it's all under control, medical obs are good, and he's driving again fine. There's a good chance he'll retire with 50 years of experience as a driver.
Somewhere in the middle are things like redeploying people into non safety critical roles if they have developed medical conditions which are incompatible with being a driver; or non-dismissal disciplinary action to give a driver a chance to come to work better rested, etc.
Of course, we have very little detail of this particular case, what the causes are, what actions have been suggested, what options have been explored and how both sides have responded. Am sure it'll all come out at some point.