Suffolk here so I know exactly the kind of thing you're describing - that low, heavy movement in treeline that just doesn't sit right with you. We don't have bears in the UK obviously, but there's been serious documented sightings of large black felids going back decades. The Beast of Bodmin gets all the attention but honestly reports like yours from more eastern counties are quietly stacking up.
Was it moving slowly or did it cover ground fast? Big cats tend to have that fluid, almost boneless way of moving, very different from a dog or fox. Also how big are we talking - chest height to a large dog, or bigger?
I'd genuinely go back in daylight and check for prints or any disturbance in the undergrowth, particularly broken lower branches and anything that looks like a kill site. Take photos of anything you find even if it looks minor. Witness descriptions alone don't go anywhere but physical evidence is a different matter entirely.
Would love to hear more detail on this one.