Don't even need a gauge, just get a tyre tread card for free from motor World or somewhere and use that as it has a ruler.
Don't even need a tyre tread card, just use the markers on the tyre.
Edit: And a spare, if present, must be legal. If it's not there, you cannot fail because of it not being there.
Plus most places AFAIK don't look at the spare.
They don't have to, AFAIK they can only give you an advisory for an illegal/bad spare.
http://www.direct.gov.uk/en/Motoring/OwningAVehicle/Mot/DG_10016070
I'm pretty sure I had an MOT failure for a bald spare iirc.
Best thing to do is to remove the spare if its border line.
THE GOVNERMENT WEBSITE said:Wheels and tyres - condition, security, tyre size and type, and tread depth. Spare tyres are not tested.
Did you even read the link?