I'll offer my 2 penneth worth on this one, as a (recently retired) M.O.T. tester. The rules were (and are, as far as I understand) that a car can be driven to a PRE-ARRANGED M.O.T. test appointment if it is "out of test", providing it is not in a dangerous condition. An example of this would be driving a car to a pre-arranged M.O.T., and discovering that it fails for a defective rear fog lamp. This is not a "dangerous" fail. In addition, the car must be taken by the shortest and most direct route; driving from my home in north London to a test centre in north London via Bristol would not be acceptable!
Once tested, there are 4 options: 1:Pass-no problems. 2: Pass with minor defects and advisories (things which might need looking at before the next test; say a tyre with minimum tread) 3: Fail with major faults noted, which may make the vehicle unroadworthy (defective fog lamp, headlight alignment incorrect etc.) and 4: Fail, with dangerous faults making the vehicle unroadworthy (rust less then 15cm from a stressed point of the vehicle frame being but one of dozens)
Defects (both major and dangerous) must be repaired before a test certificate can be issued. However (and this is the clever bit...) if you have your car tested in the calendar month before its expiry and it fails for a major (not a dangerous) fault you can still drive it until the expiry of the test, providing it's not unroadworthy, thus giving you more time to arrange the repair. That said if you take longer than 10 days or remove the car to another garage other then the testing one you'll pay for another test and leaving it at the same garage for repairs doesn't guarantee you a free re-test, as it depends on what it failed on. I was always generous with this. Failure for a dangerous fault means the car must not be used until it's fixed. My boss once threatened to call the Police when a customer said he was going to drive his car away to another garage for a 2nd opinion....
In summary, if your car, classic or otherwise, fails the M.O.T. for a dangerous fault it is deemed to be unwoadworthy and must be repaired immediately. If you rely on the classic car M.O.T. exemption two continue driving your car on the public highway and you have a crash or other accident, I would consider your insurance would be invalid, particularly if it were later discovered your car had failed the test on a dangerous fault.
On a personal note I would just add that I didn't agree with the exemption for classic cars and I don't know a tester who does. The introductory wording on the M.O.T. tester's manual (used to have) has this phrase:
"Any vehicle maintained in accordance with the manufacturer's instructions and service schedules should have no problem in passing the M.O.T. test, appropriate for the vehicle's age and design." This sums things up: look after your car as you should and your car will be O.K.!
Sorry to have gone on a bit, but I REALLY think this M.O.T. exemption for classic cars will come back to haunt those of us using, repairing and driving them!