In fact, that's what I tried initially. Both the app and the website would return and error saying something like "We regret we are unable to quote the fee in the change..."
So I figured I would save the trouble calling and just show up at the counter, only to be told no. They claimed there was a lack of something in the ticket to say it's a "Plus fare allowing same day change" or something. They went all the way to the screen to provide me a quote of £180 or so to change, and of course I refused.

FWIW, it's wasn't Expedia UK and the ticket was issued outside Europe. It's a PLUS fare regardless accordingly to Expedia and also says "BRITISH AIRWAYS PLUS FARE BETWEEN UNITED KINGDOM AND EUROPE" in the RULE APPLICATION section.

The call agent wasn't able to just revalidate the ticket either for the change, and had to reissue it in the end.
I should probably have given in and called when the website told me no, LOL