Not only must you have a bare minimum of 1.6mm of tread across 75% of the centre of the tread but your tyres not have any cracks or splits.
To help you with tread depth, nearly all tyres have a band of raised rubber running across the tyre called the depth gauge. When the tread is level with this bar the tyre will need replacing.
Incidentally, the spare tyre isn’t a part of the MOT test.
