Do I now have to purchase apps to watch previous shows on cbs, nbc, etc?

I have no idea being in the UK, but surely it should be pretty obvious to you whether you need to pay or not. Did you try before asking the question here.

CBS definitely went to a pay for mobile service quite a while ago. You can still watch full episodes of some of their most recent episodes free on a computer, but subscribing unlocks more episodes as well as mobile viewing - and they must have the Apple TV under that category as well.