There's nothing* stopping broadcasters from getting the rights to shows they previously broadcast though.
* in the vast vast majority of cases
True, but there's the question of costs and whatever and whether they consider it worth it. Obviously it's a lot easier and cheaper to show something they own.
Maybe Drama can finally show the 1989-97 episodes of The Bill now there's more of a captive audience to watch them
A shame Fremantle don't own any TV channels in the UK which would give an easier way of showing Thames content without anyone else having to see if it's worthwhile to buy it in.