If it was my car, I would very politely be kicking up a fuss. How can brakes worn to that level be considered 'approved used'? I would have expected this to be picked up in the checks and for them to have been changed before the car was put up for sale. Even if they then have to factor this into the price, bearing in mind they only have to do so at cost. £600 obviously doesn't cover it and as car is only going to be 2/3 years old at most, I wouldn't be taking it to an indie.
My own experience, and no doubt that of others here, is once they have made the sale, they try and fob you off. I would be writing to the salesman with the manager cc'd seeking clarification. At the same time, I would let them know that I am also writing to MBUK customer services. They should be able to specify exactly what you might expect as an acceptable level of brake wear. This should have been picked up as part of the checks and seems unreasonable for you to have to change the brakes having then driven virtually no miles.
I bought my own car last March, and as it was lockdown, I didn't actually see it until the day I collected it. Couldn't really inspect it on the forecourt but got home and noticed a small ding on the passenger front door. Salesman said get a quote and they'd pay. £400. They then proceeded to drag their feet and not settle. I then wrote to MB UK customer services and money was credited to my account the very next day. I get the impression they just see what they can get away with.
Back in the day they would give you a copy of the checklist so that you could actually see what is what. I don't know if they have actively stopped doing so, but there is no way they could then get away with selling a car with worn brakes as you'd immediately object.