You are better phoning up BA (don't use the 0844 number from a mobile or you will need a second mortgage). Alternative numbers are available. Don't try and call the status lines unless you have status, they ask you to verify it now.

I see n reason why the fare is not available because if you strip out one of the shorter flights you get similar options on other OW carriers. Commonly phoning BA will give you a crazy price because they need to book things like the cathay flight in an expensive class.

EG. My WT+ flight form GLA-LHR-HKG-TPE then TPE-HKG-PEK then PEK-LHR-GLA in a couple of weeks was £2,600 but I dropped the TPE-HKG-PEK leg and save £900. Then book that leg direct with CX for about £180 in Economy flex or £220 in PE flex.

Call BA and see what they can do and ask them if one leg is a crazy price. Look at the linking sectors to get a rough price in advance and if the BA price is complementary then go for it.