Have they charged you extra for it?
If not, I wouldn't worry as it's probably just a promtional offer.
@Hamba If you want to discuss this with customer service, below is the advice from an O2 Guru on how to get through.
Call 202 from your mobile.
When it comes to the reason you are calling, stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
Other ways to contact customer service are here: Guide: Coronavirus Community Help and Support