I don't think they are valid for any reason now.
From dvla website (
https://www.gov.uk/government/news/driving-licence-changes)
"
What to do with your paper counterpart
The paper counterpart to a driving licence has had no legal status since 8 June 2015. You should destroy yours if you have one, but keep your photocard driving licence."
The last 3 times i've hired a car in the EU they haven't asked for the code at all. Once you are setup and registered it's quite pain free to generate a new one. With the fairly new EU data roaming regulations it's even quite cheap and easy to do it at the hire care counter if it were ever asked for.