Thanks for the information -- and warning -- about this fare class. As a result, I went back and checked a ticket I had for weekend travel between LHR and BRU back in April and it turns out to be a V-class ticket: £69 plus £25 in taxes (!)

It reads "Valid only on BA. Non-Endorsable. Non-Refundable. Valid only on flights and dates shown".

So I guess I have bought and used such a ticket afterall. Thanks for the enlightenment and sorry about your predicament. It seems it is the phrase: "valid only on flights and dates shown" that is the operative restriction for these fares. AC's and CP's V and L fares (the lowest in North America) indicate "changes restricted", but don't go as far as BA's.