Two issues here, I do realise that I'm late to the party here though and it's irrelevant but here goes.
Do the tyres need replacing yes 100% without a shadow of doubt, most organisations recommend replacement at 3mm for good reason.
Should they have failed MOT, well based on the photograph no they should not
"At least 1.6mm throughout a continuous band in the centre 3/4 of the tread and around the entire circumference" is the wording and the current tyre law in the UK.
There is no requirement for the tread outside of the central 3/4 as outside of the main water evacuation channels some features of the tyre will be designed to wear out quicker than the main tread pattern.
So the OP was correct in my opinion and the tyres aren't technically illegal, he has however done the correct thing by replacing them