Even the cheapest s/h fares are flexible, it's just that the change fee plus potential refare on day of departure means that buying a handful of cheap O fares well in advance may do the job just as well, for a lot less money. Even last-min RFS on the day of departure looks more attractive than semi-flex!

The cheapest AMS-LON o/w BA fare (not HBO) appears to be about £25 ai.

The cheapest o/w semi-flex fare appears to be £209 more than this, coming in at £233. If you're booking ahead, outside of peak travel periods, who on Earth would pay for semi flex?

This is no doubt due to BA's schizophrenia about being a LCC vs trying to be a full service/premium/legacy/whatever carrier.