I noticed on a forum someone was asking about what would likely happen to a test if the examiner spotted damage to the sidewall of a tyre before setting out.
On the UK mainland, quite simply, a tyre:
…should have no less than 1.6mm of tread across the central ¾ breadth all the way around, and it should not have any cuts or bulges in the sidewalls.
That’s the official specification. It stops short of advising you what to do when you have an 
iron spike or ice pick sticking in your tyre. I guess they assume that common sense dictates you get the problem sorted.
If your tyres don’t meet this specification then the examiner is quite within his rights to cancel the test. The car is simply not safe.
And if he spots the fault, he WILL cancel it.
So it is absolutely unbelievable to read someone state that it is safe to drive on a tyre with a nail in it for 50 minutes – even “at speed” – because it won’t lose air, and a test shouldn’t be “lost” because of it.
Almost as unbelievable is the statement from the same person that Kwik Fit will always tell you you need a new tyre and won’t fix punctures.
That is more complete rubbish. Some branches might do that – which is a risk at ANY tyre shop – but Kwik Fit around my way are happy to fix punctures, and at least one other person on that same forum has said the same. My tyres are covered as part of my lease arrangement, and so I’d prefer a new tyre if I get a flat – but Kwik Fit have more than once opted to repair it.
It’s hard to believe sometimes that such dangerously misguided or even libellous advice can come from driving instructors.
If you have a nail in your tyre then you must get it fixed. Driving at full speed knowing you have one in there is insane, and it’s arguable whether you should be let out alone – never mind be allowed to teach people to drive – if you choose to do so.
For the record, a nail (or any other foreign object) stuck in your tyre is non-standard. It could move and cause the tyre to deflate at any time, and you have no control over whether or when that will happen. At speed, the damage it might have already done to the tyre’s structure – and about which you can only guess – could result in a catastrophic blow-out.
To suggest that it is safe to drive for ANY distance, let alone “at speed”, with such a tyre is too stupid for words.