......... its either safe for the next 12 months or not
How's that going to work then?
eg Your car passes it's MOT, and as you drive back home a spring breaks. Your car has 12/13months ticket but it's not road worthy as far as a MOT is concerned. There are far too many variables on a car to make a car not safe/MOT pass'able.
Absolutely. Just like any kind of testing, it only proves anything at the time of test. Not all testers are infallible either, so it's not even a guarantee of that.

To pronounce a car roadworthy for 12 months is nonsense.
Biennial MOTs would be a disaster, IMHO. The majority of car owners I know bar the more meticulous enthusiasts that gather at this place and similar only have their car maintained in response to MOT failures, particularly if they own older cars where "it's more than the car's worth" to change the tyres, fix the ABS, change the oil, replace the brake lamp bulbs, etc.
If you have that kind of attitude a life of public transport should be imposed on you, IMHO, but at least an annual MOT provides some assurance that a car is, if not road worthy for the rest of the year, not a total death trap.
Consider also that the bar would have to be raised for all of us. MOT testers will need to ask "will that last 2 years?" so we will not get away with presenting brake pads that have another year's wear on them, for example.
I wouldn't be surprised if the surge in dangerous vehicles then prompts a knee-jerk requirement to have vehicles serviced by a "registered" person (probably calling himself a "motor engineer"

).
Owners of condensing boilers will already know where that leads.
No, it's a very bad idea.
Kevin