ATC's reaction will partly depend on how they are recording the clearances they have issued.
In the Gatwick SID example London TMA controllers are still using paper strips. In most cases they won't check the Mode S unless they have a particular need to do it. Frankly they are usually too busy to worry about checking everything the pilot does beyond the traditional read back.
In other cases, such as Maastricht, the controller will enter clearances directly to the system which will then monitor the Mode S/FCU data and produce an automatic alert if there is a discrepancy between the two. i.e controller clears FL330 - pilot enters FL350.