The YouTube page says that this is "Psalm 62 sung by Westminster Abbey Choir at the 70th Anniversary Service of the Battle of Britain." That would have been in 2010.
They are singing only verses 1-8 of that Psalm - but here's the whole thing, from the 1662 Book of Common Prayer (Coverdale) Psalter:
As always: if anybody knows the composer, I'd be grateful....
Psalm 62. Nonne Deo?MY SOUL truly waiteth still upon God : for of him cometh my salvation.
2. He verily is my strength and my salvation : he is my defence, so that I shall not greatly fall.
3. How long will ye imagine mischief against every man : ye shall be slain all the sort of you; yea, as a tottering wall shall ye be, and like a broken hedge.
4. Their device is only how to put him out whom God will exalt : their delight is in lies; they give good words with their mouth, but curse with their heart.
5. Nevertheless, my soul, wait thou still upon God : for my hope is in him.
6. He truly is my strength and my salvation : he is my defence, so that I shall not fall.
7. In God is my health, and my glory : the rock of my might, and in God is my trust.
8. O put your trust in him alway, ye people : pour out your hearts before him, for God is our hope.
9. As for the children of men, they are but vanity : the children of men are deceitful upon the weights, they are altogether lighter than vanity itself.
10. O trust not in wrong and robbery, give not yourselves unto vanity : if riches increase, set not your heart upon them.
11. God spake once, and twice I have also heard the same : that power belongeth unto God;
12. And that thou, Lord, art merciful : for thou rewardest every man according to his work.
[EDIT: Scott comes through again - and with lots of informational tidbits! "It's a double chant in E-flat by William Boyce (1711-1779), MusD (Cantab); Organist of the Chapel Royal, 1758-79; Master of the King's Music, 1755-79. Conductor of the Three Choirs Festival and the Festival of the Sons of the Clergy. Buried in the crypt of St Paul's Cathedral [London]. Credit: John Scott's New St Paul's Cathedral Psalter."
Scott is, truly, a Jolly Good Fellow!]