As above, plus 1.6mm across the central 3/4 of the tread on a continuous band around the tyre. If your spare is no good, take it out. You can only be tested if present, and it doesn't need to be for the MOT test. Likewise, you could replace a bad one on the car with the spare, if good, and not submit the bad one as a spare for testing.
No need to shout, mdoo - it is perfectly true that on some vehicles the front and rear tyres may be of different sizes, but each pair must be of the same size and construction.
No, no need to be the same make, you can have a Michelin on one side and a Dunlop on the other.
Tyres need to have minimum 1.6mm tread across 3/4 of the tyre with the remainder having at least 1.0mm of tread and no bald patches.
Cross ply tyres are very rare now, but you cannot have a cross ply tyre and a radial tyre fitted on the same axle.. you can have cross plies at front, radials at rear (and vice versa).
You can have larger tyres on the back than on the front (or vice versa) however you cannot have different sizes on the same axle.. E.g. 165/70 x 13 and 185/70 x 13 or 165/70 x 13 and 185/60 x 15 on front. I would also think it wouldn't be allowed to have a knobbly 4x4 type radial on one side and a low profile sports type radial on the other. The tread patterns should be very similar.. not blocks on one side and grooves on the other.