The tickets are flexible only to the extent that they can be changed, but they still have to be used in sequence.
Over Xmas and during the strike period(s), when one of my sector 1 flights, i.e. Europe to LHR, was cancelled, BA agreed (very reluctantly) and I have kept a record of the phone conversation and ticket locator, to permit the sector 1 flight to be used out of sequence. My discussion was that I had paid for 4 sectors and on the basis that BA cancelled sector 1, I did not want a refund, but wanted to use the ticket.
BA finally agreed and let me use the europe to LHR at a later date. However, 2010 did have quite a few unique situations and I would only argue this in the future if a flight was cancelled.