It does appear there is no free/cheap SDC option on long haul as you say.

This weekend I tried to change my WTP ticket when I got to the T5 lounge from the BA 181 (6:50pm) to the BA 189 (4:30pm), there were 6 seats available and I was quoted £1229!

I understand there are rules etc and I would have paid something for the convivence but that cost is madness. There were avios WTPs seats on that flight for $400/24,000 so cheaper to through away the original ticket than keeping it.