No apology will be coming c.d.
That is where you're wrong! I will apologise to a
very small number of people who have suffered theft and who have reported it honestly and wihout exaggeration.
However, British Airways is almost watertight as far as baggage security is concerned and I hate to see subjects such as "BA, Thieving B@st&rds" used, and accusations flying around, wihout a shred of evidence, to slate hard-working people who do the most horrible, nasty job in the industry!
To get back on the Original Topic, the baggage allowance is clearly stated - 1 piece at up to 32kg for domestic services. It is a bit harsh to charge excess when the airport authorities are forcing cabin baggage to be checked in, but I feel you haven't made us aware of the full facts -
How much other cabin baggage was being checked in? (I say that, as I have never, ever seen anyone travel with just a vanity case in the cabin!