Depends more on what it tastes like than on what goes into it. I'd say American. We don't really get US 2-row in the UK and it is distinctively bland when compared to most British barleys. Same with the malting. We do get extra pale MO now, that is a bit more like US 2-row, but it's not very commonplace.
Styles have always been very fluid and boundaries very blurred. American brewers have used foreign hops for a long time and so have British ones too. A pint of regular British bitter would have had an average OG of 1.055 in 1910, 1.035 in 1970, more around 1.042 now. Back in the day it would have often had cheap American hops or expensive German ones, in the 1970s mainly British ones and nowadays again a bit of everything. Most British beers don't conform to any standards anyway, like BJCP guidelines and stuff.