It's a very complicated question to answer
But basically,
Forget calling it an 'MoT Certificate' for a start, its more of a proof of inspection
a passenger carrying motor vehicle or commercial vehicle under 3.5t gross weight requires a MoT inspection after 3 years from first registration and then yearly after that, this is written legislation in the road traffic act 1988,
So as long as your car has had an inspection in the previous 12 months you are complying to this part of the law.
Then the tricky bit is the My car has failed and i want to still drive it!
Well you can,
Say it failed on a headlight inoperative, It's the middle of the day in summer,
is the headlight making it unroadworthy to drive around during the day? No.
So it failed on only one brake light working, yep still ok to drive away as you are still able to signal to the person following your intention to stop.
Ok this time it failed on a tyre that has separation of it structure causing a bulge that is likely to go pop now you can no longer continue to use it on the public highway until the tyre is replaced, this includes driving away from the test station.
Final example, it has burst a brake pipe and has lost all fluid to the rear split giving no brake effort to either rear brake, the tester will mark this as dangerous and the car can not be used until the defect is rectified and a retest done.
passing an MoT and being roadworthy is two completely different things, as at the side of the road a Vosa examiner or suitably qualified police traffic officer do not test to Mot standards, they test to 'Categorisation of Defects' standards which is why example 1 and 2 above would allow continued use of the car in certain condition.
Anyway here's the 'Cat of D's' if you want to know what deems a vehicle unroadworthy or not.
https://www.gov.uk/government/uploa...ata/file/498457/Categorisation-of-defects.pdf