Sir Michael Caine saw a lot of himself in the character of Harry Brown, they're both combat veterans (Harry is a Marine who served in Northern Ireland, Caine served in the British Army during the Korean war), and Caine lived in the same area that Brown does. It was things like these that drew him to this movie.
The woman singing "Gold" in the pub was the unit nurse on this movie.
When Harry is going through the items in his "treasure box", he pulls out a knife. It is a Fairbain/Sykes knife designed for knife fighting, especially for military commandos in World War II.
The song at the end of this movie is called "End Credits", it is by Chase and Status featuring Plan B (Ben Drew). Ben Drew played Noel Winters in this movie.