[ and whether petrol or diesel, if that is relevant ]

Passed MOT in December.

Brakes and pads came up as an advisory on service.

The garage who are telling you it needs new pads and discs are clearly dishonest. If the discs/pads were worn to a significant extent the garage that did the MOT in December would have noted them on the test as an advisory.

I would suggest you visit the garage who carried out the MOT and ask them if the pads/discs do need attention and tell them why you ask. They did nothing wrong passing the car provided they were up to stopping the car but I would be very surprised if any MOT garage did not make a note regarding pads close to the sensible wear limit.

If they suggest a change ask them for a price.

Service done seperately at a VW indie.
MOT done at a local garage as they were cheaper and had availability when I was off work.
I've done 41,000 miles.
Apparently there is some corrosion on the pads.

The pads won't rust, it's the disks but without seeing them it's impossible to comment on their condition.

The garage where you had it serviced are saying this is advisory so on the assumption it's still ok to drive I would shop around to get several prices.

7mm is quite a lot still. I've got something like 2.5mm on mine as I was advised that they were wearing thin at my last MOT (August 2021). I don't do many miles, and they've been fine up until now.

I did price new pads AND disks all around on my 2002 Honda Accord and including most of the tools needed to do the job (torque wrench, axle stands etc etc) I was looking at less than £200 (to do it myself).

I'm sure a Honda dealer would be looking at well over £800......