on-demand stuff checks a couple of things
you can normally watch the channel it comes from – so if its a movie, you need the (right) movies package… and conversely, the free-to-air channels, i.e. bbc/ITV/4od/5, are all free.
(as liftman says) you need (or certainly did originally) a minimum of the entertainment extra + package to get box sets
lastly that, it might be a pay-per-view event, in which case you pay, er, per view, all labelled as ‘sky store’
let us know if you can get to the inbetweeners, cause I’m not sure what happens if you don’t have extra+ package, but are getting a box set from a “free” channel.