Quoth Patrick Grady:

I think the song is about Britain's relationship with America. It is being sung by the pompous Brit, who looks down on his uncouth (indolent) friends, but nonetheless realises how weak his position is in the world - thus "do not forsake me". The talk of being 1000 years old lends itself to this, as many of our politicions like to talk about Britain (England) having such a long history. "Darkest hours" probably refers to wars and simlilar crises, when the UK is known to huff and puff a lot. "The flower of speech" is democracy, which we don't have much of over here, and "Misbegotten notions" are our quaint traditions like the monarchy and so on - totally out of place in the modern world, but what do you know?

Quote William Scrivener:

I always thought this song as a sort of "Sell Out" song, Meaning wise

Oh Do Not Forsake m My Indolent friends                                     
Oh Do not forsake me though you know i must spend    
all my darkest hours talking like this for i am one thousand years old.         

This Singer has sold himself out to culture and forgotten all his friends He then loses his popularity and his friends hate him AKA have forsaken him He apologizes to them and tells them that he must sing like this (which could mean giving tham a bad name) or else he won't be popular. He has been popular for a long time, that possibly seems like one thousand years.

One thousand years old, sure you'd say that's old                                
One thousand years old, but what do ou know          

His friends are saying, you've been popular too long, you're dried up, you're a has-been he's saying, what do you know, I've still got it! At the end he manages to buy out his friends (We'll be one thousand years old).