I think for me, it's the fact that it would've failed an MOT if pulled over and that it may violate section 13 here:
http://www.legislation.gov.uk/ukpga/1979/54
I can't honestly say I would check the tyre sizes when leaving a garage to check that they haven't swapped them over (ok, so it would be impossible not to notice in my case, but when I had the 18"s it would have been possible). If I'd then have been pulled over by the rozzers for something else and they'd noticed, is it right to blame the driver?
That been said, I think that you should be checking your car after every session briefly, tyre pressures, brief glance over fluids etc just to make sure you've duly checked anything that you can.
I'd be seeing what the garage would have to say before doing anything though, but if they have misfitted wheels/tyres, maybe it should reflect poorly on them. Let's say they changed something which wasn't visible, such as springs, or didn't fit some suspension parts properly, would you really be checking those?