Re my post 1031 above. I called at T-71:59:59 seeking a re-route. As I expected it wasn't simple.
Call 1 -
To general U.K. Call centre number bounced offshore.
"we can move you to Turkish."
"No thanks, can I have BA via LHR? (I give precise flight numbers having checked availability in the same class - loads. Obviously only J fares now available at T-72.)
"No sorry, not available. A refund can be only applied for through our website"
Call 2 -
To the ticket store in Harrods.
"Sorry I'm with a customer, please call back."
To be fair - they called me back 15 minutes later.
Call 3
"We can move you to Turkish or full refund"
"No thanks can I have BA via LHR (give identical flight numbers as per call 1)"
"No problem, I'll ticket it now" then waited to make sure I received the confirmation. All sorted.
Moral of the story - do your prep work and HUCA if you get an unhelpful agent.