Harry Brown.

This should work as a British companion piece to Gran Torino.

However, despite the fact that they both feature 2 elderly retired from the military type characters, they take 2 very different paths.
When Eastwood's character shifts from a racist & cantankerous old git towards someone more accepting and even self-sacrificing, Caine's taking something of an opposite direction: from quiet retired to avenging angel. Because this is a revenge movie. So it's actually closer to Dirty Harry.

Caine's Harry saves the movie from any misplaced far-right extremism though. He depicts a faillible character, who doesn't quite enjoy the path he takes but takes it anyway because he's just fed up in a sense. He's lost too much and sets himself to do something.
Thanks to that, the movie manages to navigate in this grey area that is vengeance. The "baddies" are also interesting in the sense that they're not likeable in any way (they're dispiseable thugs), but they're caught in a system, adding a relevant social commentary.

It's quite a tough movie, violent and bleak, hard to stomach at times, yet it's a fine example of what British cinema can produce.