OP.
The 0.04mm measurement will be a typo. To quote Paddy Ashdown, I’ll eat my hat if a MOT inspector in Halfords can measure 0.04mm accurately over a whole tyre tread. If they really quote 0.04mm then they are as dumb as a bag of hammers. That would be an easy fail on tyre depth, new tyre.
Tread wear indicators are normally at 2mm, although I’ve seen some at 2.5mm. The legal limit is 1.6mm over the central 3/4 of the tyre. This can then be “interpreted” by the measurer into legal / illegal. The fact that two tyres the same size will have two different tread patterns and will be different real world sizes. Much the same as bike tyres.
Ex tyre fitter, so I know the score.
Minor Rant of My Own:
MOT advisories. It has either passed or failed. You can’t have a tyre, brakes, lights, etc. be “ermm, well it’s close, but I’ll let it go”. Pass or fail. Simple.
There’s nothing wrong with a garage or mechanic telling you during or after work has been carried out that “your tyres are getting close to the limit” or “depending on how many miles you drive, you might want to check the brakes on the front in 6 months or 5000 miles”
MOT, it’s passed or failed. Printed on the MOT certificate is pointless bullshit. (/RANT)