Wouldn't surprise me at all. Somerset's had big cat reports going back decades and Shropshire's no different, the hills there are perfect terrain for something like that - plenty of cover, deer to hunt, not many people about.
My mate's dad swore he saw a large black cat near the Quantocks back in the 90s, described it as moving completely differently to a domestic cat, low to the ground and deliberate like. Stuck with me that did.
What size are we talking roughly? Labrador-sized or bigger? And did anyone else in your group see it or was it just you? Always good to get corroboration on these things, not that I'm doubting you, just helps build a proper picture of the sighting.