On a totally different forum a member who is totally familiar with MOT testing ( where as one part of his role he does or has done audits) makes frequent contributions. There are too many to record on here but here is one. I have removed the preamble.

If your car is submitted early for an MOT and it fails, (on a dangerous fail and is marked as so), the original certificate remains in force, but the one you have just been handed as the dangerous fail now means you are knowingly driving a defective vehicle, a road traffic offence. It must not be driven. Please don't confuse a fail with a dangerous fail as they are completely different, the fail allowing the vehicle to be driven away.

Hope that adds a bit of clarity.
Personally I would expect that if you knowingly drove a vehicle on the road that would fail an MOT with a defect that you considered might be dangerous you would be negligent and potentially liable for any consequences.

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