My guess is that the wild animal that poses the worst threat to a cat in the UK is probably a badger. Maybe a Scottish wildcat, though those are very rare and remote, and it sounds like those are more likely to mate with a housecat.
It does sound like the American badger, while smaller than its European counterpart, does tend to go more in for hunting, though:
Carnivore; gophers, squirrels, rattlesnakes, and birds
Primarily omnivorous; earthworms, insects, grains, and small mammals
Behavior
…frequently hunts with coyotes
Might be something of a wash there.
EDIT:
They also do have the adder, but we’ve got rattlesnakes, copperheads and cottonmouths, and I’m pretty sure that those, particularly some rattlesnakes, would be more dangerous.
The eastern diamondback rattlesnake is the largest rattlesnake species and is one of the heaviest known species of venomous snake, with one specimen shot in 1946 measuring 2.4 m (7.8 ft) in length and weighing 15.4 kg (34 lb).
The eastern diamondback rattlesnake has the reputation of being the most dangerous venomous snake in North America.[40] While not usually aggressive, it is large and powerful. Wright and Wright mentioned a mortality rate of 30%, but other studies show a mortality rate of 10–20% (untreated).[18][41] The eastern diamondback rattlesnake is a major leader of fatal snakebites within its geographical range.[42]
And that’s mortality for humans, which are a lot bigger than a cat.
Fortunately, adders tend to keep themselves to themselves and don’t bite unless they are scared.
If your cat has bitten by an adder, their symptoms will depend on where the bite is, and where the venom spreads to. If the venom stays in the tissues around the bite, it will cause pain and swelling, but if it spreads further and gets into the bloodstream, it can cause much more serious problems such as damage to the liver, kidneys, heart and nervous system. If venom is injected straight into the blood stream, it can cause death very quickly. It can also be very serious if your cat is bitten on the face, tongue or neck because swelling can lead to breathing problems.
With treatment, most cats make a full recovery, but sadly, in some cases, adder venom can cause severe illness and even death. The quicker your cat is treated, the better their outlook. Any cat with a suspected adder bite should be seen by a vet ASAP.