The 10% discount offered to holders of 200 BA Shares is going away from 1 January but you should be able to book for now. I tried a simple trip from London City to Dublin. Whilst the first page shows the correct, discounted, price as soon as you go through to book the system the system reprices it at full price.
Has your discount stopped working?
The £12 difference is significant so I’ve emailed BA. Let’s see if they reply before the discount ends.