It all depends if the vehicle you're driving received type approval after a certain date.
Before that date DRL's were optional, after that date they were a requirement - wasn't it February last year?
Anyway, cars manufactured after that date still have optional DRL's - it's only new cars/models that require type approval.
I'll use the Skoda Octavia as an example:
Octy 2 manufactured before cut-off date - DRL's optional. Can be switched on/off
Octy 2 manufactured after cut-off date - DRL's optional. Can be switched on/off
Octavia 3 type-approval gained after cut-off date - DRL's compulsory, should be no way of switching them off.
The majority of cars on the road right now you should be able to switch off. However as the new models arrive they will be compulsory.