I got £24k using the figures you gave:
If BA had kept their word I would have retired at 55 and started receiving a pension. Over the next 18 months this would be worth £75k (it will be much less than this but let's run with it!). If they force me to keep working, I will not receive this pension, but will earn £110k. Therefore, for working for 18 months I have received only £35 k.