If a mezzanine within a flat is removed - will the price of the flat fall as a result? The floor plan seems to count those sq. m. into the gross internal area.

Do you mean will the value be lower when you sell it compared to if it had the mezzanine?
It depends not just on square footage, but also on appearance. People with small children hate the open railing. Plenty of people think they are wonderful.
Ask a real estate agent to gauge all that in person. It's not that simple.
How many square feet? How long do you plan to live there? How much do you dislike it? While having one eye on resale, don't forget this is your home first.