Saw and Saw II are occasionally shown on Channel 4 and Film4.
Saw III and IV are currently being shown on Sky Movies, but Saw III has been on there for quite a while now so i'd expect it to premiere on C4/Film4 sometime later this year (probably before the cinema release of Saw VI). Saw IV only premiered on Sky a few months ago so i wouldn't expect that on terrestrial/freeview for at least another 18 months.