Nothing will happen, even if they know the cars that shouldn't have passed.
A MOT certificate cannot be withdrawn, I found this out personally.
I bought a d reg sierra auto, when checking it over/servicing a bit closer the day after I found 7 major fail points, 5 of them structural.
I gave the garage the option of correcting at their cost the rust, but they told me to do one.
I informed Jean Paul from the MOT inspectorate in Norwich Norfolk, long story short car inspected agreed it should have failed, badly, tester called to do 30 mile journey to view the car at the garage where it was on the ramps, not happy he attended, got his bollocking.
I then questioned the validity of the MOT certificate in light of this, he said and I quote.
"the certificate has been issued, it cannot be withdrawn, you can use it till the next mot"
I questioned what if stopped by plod, he said "nothing to do with them, it's not unroadworthy or dangerous".
tabetha