This is Jerusalem, based on the poem by William Blake and the music by Sir Hubert Parry, it's one of Britain's anthems, though I personally prefer it even to the actual national anthem.
Small aside, I've only heard this in person once, and it wasn't at all expected. I was at St Paul's Cathedral in London, it's a great place to visit if you ever do come to London. I spent a few moments taking in the sights at the cathedral floor, that looks like this:
I then made my way up the various levels to the top of the cathedral, some 500 steps, magnificent views at the top at the golden gallery. Upon making my way down I heard the sound of a distant organ being played.
I realised that it was the main organ pictured in the photograph above, to the left of the hall that stretches in. I rushed down to the cathedral floor as quickly as I could, since I adore organs. Alas, as soon as I entered the cathedral floor, the organ stopped playing. A few seconds later, they started playing again, and they played that song, Jerusalem.
It was quite something, it's a mighty organ, 4th largest in Britain with 7,000+ pipes!! That combined with the sights of the floor at St Paul's. The cathedral also has a massive dome. So the sound resonated and built up magnificently, your footsteps alone would echo, just imagine the sound of a mighty organ.
That was a memorable experience, the likes of which I didn't expect.