your tyre will go down,either slowly or if the nail gets moved about, quite quickly! If the tyre is going down slowly, it will affect the handling of the vehicle, if it deflates quickly and you are going at speed you could be in serious Sugar, even if you aren't travelling at speed if you end up driving on the rim of the wheel, it can damage it so that you would you need a replacement wheel as well as the tyre.
http://www.ctyres.co....info/Tyres%26MOT.html
dunno where you got the MOT done but a nail is a fail:
"Tyres fail if they have serious cuts, bulges or other damage. The wear on the tyre is checked with a tyre tread depth gauge to ensure compliance. The tyres are examined to ensure that there is no fouling with any part of the vehicle." - I would have thought a nail counts as "other damage" at least.