Feral pigs?
My brother?? 
Seriously, thinking about it, all that comes to mind are the rumours of various “big cats” (possibly panthers) which have supposedly been spotted but never quite proven.
Julie
It’s the badger. Sad but true.
Depends what you consider as “wild”. Genuinely wild badgers are nasty if cornered, but won’t spontaneously attack you: the badger that recently went on the rampage was one that had been in a sanctuary and lost its fear of humans. Stray dogs gone feral and in a pack are pretty nasty.
Drunken soccer fans?
In contrast, Spain has bears, lynx, vultures, eagles and some rabid politicians.
There’s the adder, Vipera berus
Must be weird to live in a place where there aren’t deadly snakes, spiders, jellyfish and octopuses everywhere. 
When I read this sentence (before looking to see who was writing) I immediately thought, “Oh, this poster must be from Australia.”
… Or Disneyland.
Julie
I was going to say badger as well, although IANE. The question doesn’t stipulate who it is dangerous to and I belive badgers are at least blamed for a lot of damage to livestock, spreading disease etc. Otherwise it must be foxes, takes a dozen blokes on horses with guns dogs to take down oen of them!
Yeah. Where I am, in Sydney, it’s difficult to ever get more than a few meters away from the nearest Red Back, Funnel Web or White Tailed Spider. I know with certainty that there are at least hundreds, if not thousands, of Red Backs and White Tails within ten metres of me right now.
How about deer during the breeding season?
Addersare pretty pathetic, your not going to die if your bitten by one unless you have an exceptional medical condition. I’d vote for badgers as if you get on the wrong side of one of them it can rip you to shreds with it’s powerful claws.
Of course you do get the odd dangerous shark and other animals such as killer whales in the sea around the UK.
Isn’t there some parasite/predatory worm annihilating the earthworm population in England? (I believe this to be true because I saw a documentary on it a few months back) Given the role of the lowly earthworm, this could be construed as insidiously dangerous…especially if you’re an earthworm!
Bears
We only have bears in zoos nowdays, I think the wild ones emigrated to the new world on the mayflower.
You won’t find many Alien Big Cats here, but some have been captured or found dead- they are usually tame animals which have been released or stolen, but in some cases have been living in the wild for some months.
In scotland there is the Wildcat, which hybridises with domestic cats to produce the black Kellas cat- both wild and hybrid are nasty pieces of work.
But most sightings of alien big cats are misidentifications of domstic cats, labrador dogs or foxes.
Vipera berus bite is nothing to joke about: here in Scandinavia the latest lethal bite happened in 1998, the victim was a healthy adult man. Another 30-year old man spent 2 weeks in a hospital bed after a bite.
The severity of the bite can range from slight swelling to excruciating pain to death, it all depends on how much venom gets into your bloodstream and how your body will react to it. There’s no way to tell what will happen beforehand.
Anyway, I’ve encountered numerous adders in my woodland treks and never seen one that didn’t try to get as far away from me as possible as soon as I got to within a couple of meters of the animal.
I think it’s the Big Cats - which are rumoured to exist in Cornwall and Scotland (and if you know your geography you’ll laugh your ass off at that oh so obvious connection)
If not then I would plump for wild horses or cows. They can be pretty vicious.
Feral dogs and cats - strays basically. They can be quite dangerous.
We’re very tame here in Britain - lol
If you want to extend the question to include Scotland then the two most dangerous animals are the wild cat - this looks like a domestic cat but is unrelated ,is very aggresive and cannot be tamed even if you get hold of a very young kitten . The other contender is the polecat ,which is a type of weasel or stoat. These also can be dangerous. I think that this second animal might also be present in the north of England.