Interesting one. I'm based in the States but I've been following the Shropshire big cat reports for years - there's a surprisingly consistent pattern of sightings in that area going back decades and the descriptions don't vary much, which is actually more compelling to me than if everyone was reporting something different.
The melanistic leopard theory is the one that keeps coming up and honestly it holds more water than people give it credit for. The UK had pretty lax exotic animal ownership laws before the Dangerous Wild Animals Act in 1976, and a breeding population could theoretically sustain itself if the terrain is right. Shropshire hills have enough cover and deer to support it.
What I'd want to know from anyone who actually saw it - what was the light like, how long did you have eyes on it, and did you get any sense of scale relative to known objects in the same frame? Those three details separate the credible reports from the misidentifications in my experience. A big dog in poor light fools people more often than they'd like to admit.
Anyone got photos?