Let's get something straight here - the short-haul fares nowadays are comparable to EasyJet or Ryanair, with some being even lower than the charterers.
Nobody bats an eyelid when EasyJet or Ryanair charge for every grramme of excess (and their allowances are significantly less than BA's new ones), but somehow you feel BA is being unreasonable when it enforces the allowances.
The US carriers have disallowed bags over 23 kgs for over eighteen months now (AA, CO, US, UA etc) and charge $50 if a bag is over 23 kgs.
BA has chosen to have 23 kgs as the absolute upper limit - our prerogative, end of story! The terms and conditions of carriage are perfectly fair and reasonable and sufficient notice has been given for you to find an alternative carrier if you don't like the new allowances. The vast majority of passengers will notice little or no difference. The "traders" who have made vast profits at the airlines' expense for so long will now have to pay their way - and what's wrong with that?
If you ask for a refund of an existing ticket because the changes to the Terms and Conditions are no longer acceptable to you (ie You use the Trading Standards Terminology, asking to speak to a supervisor in necessary, that the reservation is "Not Fit For the Purpose For Which It Was Sold!") that we will undoubtedly refund your ticket price. British Airways usually acts fairly and reasonably under UK Law.
The aviation industry is undergoing rapid change. As fares are being driven ever downwards, managers are being forced into lateral thinking in order to replace the lost revenue.