Let me explain. When I worked at Hampden House in the 1990s power failures were a regular part of life, the power lines in rural Buckinghamshire are (or were) terribly unreliable, especially in winter. Now near the beginning of Silent Scream there's a scene in Chucks' house where the lights go dim during a storm, why? what's the significance? None that I can see in the movie as released. I think that the original ending was the couple having given up all hope of escape, are released by a power failure, note the rickety overhead power line shown near the last scene, wouldn't take a lot for that to fail now would it? On the other hand our nasty little pet shop owner would have the benefit of a decent national grid supply in town!
It could just be that Hammer wanted to subliminally suggest a possible escape route to the audience with the dimming light, but that would have been lost on anyone not used to rural life.
No doubt if it had been made in the US such an ending would have been used just if they had made the Wicker Man we would have seen helicopters arriving at the last seconds to rescue the burning Ed Woodward.