The UK tends not to do consistent gradients in the way that they do on the continent
That’s why I suggested those two. Not that they are the biggest or steepest*, but they are the most alpine-like in character. Coming from Port Talbot past Afan, you start off in a town , wind your way up throuh valleys, woods, the odd village, then the trees thin out, it gets all bleak, and you finish at a proper pass after about 13 miles. Even better if it’s a dull day as you get into the clouds and wind-whipped mist at the top and you can see bugger all. Last 5km or so is about 5%.
* Afan one has about 520m ascent, so it’s not as big as the alps but not trivial by their standards either.