I am no expert but my guess is the BBC agreed to move Dave so that the deal could go through for less. If they wanted EPG
12 then it would have cost them more.
I guess they done this swap to tie up the loose ends before the deal completes. If the deal was done then it would have been difficult to move channels because they are two separate broadcasters and that probably goes against a Freeview rule and it stop anyone squatting or making money out of someone else channel place.