As you have already started your journey, you should be able to deal with BA rather than AX, but that is not the real problem you have.

The fare basis you bought required a Saturday night stay. These are generally cheaper as many travellers, especially those on business, want to be home for the weekend. This way mid-week fares are significantly higher. Anytime you stick in a Saturday night stay, the fare comes down.

It is possible (depending on the exact fare rules on the T fare you bought) that you can change this, though this might involve paying a change fee and the (probably very high) fare difference to the next fare class, if there is any availability in it.

Unless you need to be home, enjoy the tennis!