If you've downloaded content with, say, the BBC iPlayer app (on an 'offline' basis, rather than a streamed one) you'll then be able to view that content on your iPad anywhere until the licence period expires. (e.g. if you download a programme today that's only got 5 days left to view, you'll be able to watch it on a plane tomorrow but not next week because it will automatically have been deleted from the app by then).
As far as airline security rules are concerned, you can take an iPad into the cabin of a plane as long as you're not flying to the UK from Lebanon, Jordan or Saudi Arabia. (A few airlines might have their own restrictions on what counts as hand luggage though).
Electronic devices which don't try to connect to mobile phone masts can generally be used at any time during flights except during (and immediately after) take-off and immediately before, or during, landing.