Its so not a stupid question and one that was debated and debated in my film class.
"Harry Potter" as an example. Its set in Britain, has a British author, British characters and mannerisms, but in my opinion isnt really a British film. All the funding, advertising and distrubution comes from the U.S. All the control is coming from the U.S., and that to me doesnt make it a British film, but an American film.
Something like "Love Actually" is not entirely American, but its not a British film. Although a great proportion of the funding comes from the U.S., the control is had by Working Title, a British company. They also have the inclusion of an American Star, and also many schmaltzy and optimistic themes usually found in sappy U.S. comedy
To me a film like "Dead Man's Shoes" is what is best described as British. British director, British star, British setting, British funding, British values. It was made inclusively and indepentantly within the country, with only help from the U.S with distribution later on.
I dont know whether being inclusively British is good or not. Some of it is brilliant, some of it is boll***s
My favourite British films:
"Bullet Boy"
"Dead Man's Shoes"
"Once Upon a Time in The MidlanRAB"
"A Way Of Life" - Seriously everyone has to see this film. SERIOUSLY. Its so good, and nobody has seen it. Cant recommend it enough