On any car if you get a check engine light or other fault light this typically means one or more of the sensors has failed or has a faulty connection.
The ECU senses this and may put the car into 'limp home' mode which typically limits the revs and the amount of power available. Usually the car is safe to drive for a short distance as the mode is intended is intended to get you home without incurring any engine damage
The quickest route to repair is to have the car plugged into a diagnostic machine which will read off the fault code stored in the ECU (providing the light is still on at this point) and indicate which sensor has failed or whether the fault is more serious.
You can do a very rough and ready check yourself by unplugging sensors one at a time and seeing if any of them change the driving condition which would indicate the most recently unplugged sensor is the faulty one. This method is not foolproof though and reading a fault code off is the better method.
from your description it soudns like the fault is minor as you still have power enough for the motorway
so it possibly something like an air temperature sensor but really save yourself time and money by getting the diagnostic done 1st