[ASnep]

No because 12422 isn't the BBCs it's someone else's, they just use it occasionally. 10803 is their space uplinked by them

Just because they don't own it now, doesn't mean they couldn't aquire it and hand back 10803 to SES in return. And who uplinks which transponder isn't fixed as well, if they'd keep it they could uplink it themselves no problem.

When the SD services end next year the BBC will be able to stop paying for a whole transponder. Renting the whole of 12422 just to fit in one service would cost more than just renting the small amount of space from Sky. Also by this time next year, it might well be that there'll be more space due to BBC4 and CBBC closing

That's why I have mentioned 12422 being a wide transponder. If it is switched to DVB-S2 it could fit 6 HD channels instead of 5. So you could move the services of one of the exisiting slim transponders to 12422 and still have enough capacity to add BBC1 South as well (elliminating the need to rent capacity from Sky).
We also don't know when BBC4/CBBC are closing, that still could take some time.

They will have chosen 10803 for the nightlight for a reason. It'll presumably either be because their contract for it is first to expire, or it is the transponder that has the cheapest early termination fee.

Moving one of the existing HD transponders onto the slightly bigger 12422 would mean they'll still have a spare BBC owned transponder without anything on it, they currently only need 5 and a bit

Again you can (like you pointed out yourself) terminate the contracts with SES for transponder rental. 10803 and the other TPs currently used aren't perpetually linked to the BBC. And my point is they only need 5 TPs, with one of them being a wide one, to fit all services.