Well both Sky AND BBC/ITV have a service called Freesat.
This is where you buy the box, and watch certain channels via your dish, but pay no monthly fee.
You cannot of course then add the movie channels.
As you already have a Sky dish and box you can buy a Sky Freesat card for �20 I think (one off payment).
There would then be NO monthly payment.
Sky say you can get 200 channels (no idea what those 200 are though).
Ring up Sky are say you are interested in their Fresat, they may even offer you a "deal" to keep the movies, but maybe at a reduced cost.