The Monster Squad is a fixture on sky movies,but it does come on terrestrial/freeview,usually once every couple of years.It's normally on Five.
Near Dark had a run on ITV 4 a while back.
Not really great movies,but A Nightmare on Elm Street 3,4 and 5 have never been shown on terrestrial television.I find that quite odd considering how popular that franchise was,and still is.
The once outlawed horror sequels,Texas Chainsaw Massacre 2 and 3 have never been shown on terrestrial or freeview.However,both are fairly trashy,yet not as crap as the 4th movie which is sometimes shown on ITV 4.