Looking for a British crime film

Dear community! I remember seeing a British crime film that I cannot seem to find again - please help!

The film starts with a rich landed family getting introduced to their posh daughter's new boyfriend who somehow hardly fits in. The tension quickly grows, until we learn in a flashback that he is not actually her boyfriend, but had kidnapped her on the country road to her parents' estate. He is one of three (?) working class guys who decided in their pub to invade the home of rich people just to confront them with their vulnerability. One of them does it purely for ethical reasons, another wants to steal and take something, like the father's expensive watch collection. (I remember his rant about rich people always having an extra safe for when their home gets invaded.) Tensions escalate amongst the criminals over how to proceed, and after a bloody climax they basically all wind up dead.

Does that ring a bell with anyone?