Low budget post apocalyptic British?

A relatively recent film I think. Two brothers barely surviving in a post-apocalyptic world, living in a cave and scavenging as they await the return of their father on a remote beach. It soon becomes apparent that the father isn’t coming back and one of the boys sets out to find him; cue visiting abandoned towns and army bases, avoiding marauders etc. I think that there might have been a female character showing up to provide a different dynamic? I think it was set in Scotland – I’m pretty sure it was a British production, something of the feel of classic b/w British sci-fi about it, although it is a colour movie… definitely not Doomsday!