The reason i state about reading all the posts is simple, there mayve been a nugget of info mixed in there that might help, however your post did state that this was one linked issue, and as answered, the channel automatically switching to 799 is nothing to do with the BBC pulling channels and making changes, but the rest of it is simple, switch to freeview until all the issues are resolved, have you contacted SONY??, perhaps your tv has a fault, mines tuned to freesat and never changes to 799 on its own, but then ive altered my channel layout anyway, perhaps that would be worth a try. Downloading Sonys channel editor for the pc, i exported my channel list to usb, then altered the layout to my preference putting all had channels first and moving all the BBC sd rubbish to the back of the list, Saved it, put it bk on USB and reimported it overwriting the current laid out channel list.
Worth pointing out that USB also works as a emergency recovery tool, if, which has happened once, i wake up and all my channels were somehow gone, meaning id have to retune the whole thing again, No Need, just slap the usb back in the port, reimport the channels again and boom, the channels are back and all in the layout i like.
Im thinking it may possibly remove the switch to 799 doing this, whatever was on 799 i deleted, my list only has 180 channels, the rest were deleted.
so I couldn't get a code back BUT after reading comments here I'll give up as beer is more fun 
