If you're talking about Britons in general, they have no specific definition of ale, other than it's a synonym for beer, which probably doesn't include lager.
If you're talking about Britons with at least some knowledge of beer, they'd regard "ale" as covering bitter and mild, probably not barley wine, almost certainly not stout/porter and definitely not lager. Most Britons - quite correctly - know not, nor do they care whether stout/porter is "ale", although from the historical purist's point of view, the answer is that it's a beer.
Oh, and as The BeerNut indicated, the idea that "The porter purists would argue that Roast Barley shouldn't be present in a Porter and is an ingredient reserved strictly for stouts" is bollocks. When roast barley was finally allowed into beer in the UK (which at that time still included Ireland) it went into both stout and porter.