I can't comment on how things work in JNB as I've never been there.

However, at LGW (and T5 I believe) the wheelchair service is provided by an outside contractor. They don't just assist BA passengers but passengers of every other airline operating into LGW.

Normally they're at the gate waiting for passengers before the aircraft has arrived but there are issues sometimes when they're busy and passengers have to wait on board, or by seats at the top of the jetty.

If there are a number of wheelchair passengers on the flight we will ask those able to walk short distances (classed as WCHRs) if they can manage the walk up the jetty to the waiting buggy. Ditto those who can walk but can't manage stairs (WCHS's). We also have a WCHC category which is for those who can't walk at all and who would require lifting on board.

For as long as I can remember BA will not allow any passenger requiring wheelchair assistance to be seated on the UD. I believe this is also detailed on ba.com. I've been told this is due to mobility in getting wheelchair passengers up and down the stairs (obviously an issue with WCHS and WCHC pax, and frequently an issue with WCHR's), and in case of an emergency. It's the same with exit rows.

If you've been able to reserve the UD before, it's probably because the wheelchair request has not yet been put in. However, I would be thankful you have not previously been moved when arriving at the aircraft in a wheelchair as that is what should have been done.

We don't have 747's at LGW so the UD issue doesn't come into play there, but as soon as a wheelchair request goes in to a reservation using an exit row, then the passenger will be moved from the exit row.