Thank you for the quick reply. Yes its a Club Europe ticket. I just tried to look in Expertflyer the fare rules but as Airprox mentioned it's not there. I had some experience once at LHR ticketing desk where I tried to move myself to an earlier flight but the agent said I can't as my ticket doesn't allow that, etc. I remembered that when I booked it was a SDC fare option but at the airport I just couldn't do much. So I decided to look into this how I can "prove" hence my question.
I guess I just need to take screenshots of the booking.

I have only one question left. I have a booking in the future which was multiple segments (all CE) and when I booked it on BA.com I've got a different fare rule explanation. (similar wording what you find in expertflyer) Obviously that doesn't have on ba.com the wording that it's a SDC ticket. However that ticket has the usual endorsement as NONREF/-V2R. Is that mean that I still have SDC feature on the whole booking? If yes I guess the only way to "prove" it if I take a screenshot of checkmytrip and show them the endorsement...