Map: Union of the English-Speaking Peoples (1941)

I think apart from anything else it would be impossible because:
a) Its basically a US take-over of the British empire, plus Eire.

b) It would be impractical to ignore the continent of Europe. Even aside from it being dominated by the Nazis, who wouldn't be willing to leave the ESU alone, the continent has too much industrial resources and population, even without the associated colonial empires, including lands in the Caribbean to consider.

Also there is a considerable cultural difference between Britain and its dominions and the US, let alone the differences between the dominions and/also between them and Britain.

  Intersting, thanks.  He's still being isolationist in that he thinks such a union, if it came into existence, would be able to ignore events in Europe.  Plus its unclear what are supposed to happen to the British and US empires?  Are they to be defended, abandon to fascist conquest or what?  Immediate independence would be impractical in many cases plus even where it might, such as India and the Philippines, those are unlikely to be able to defend themselves without considerable support from the proposed union.  Similarly neither S Africa nor Australia would be defendable in the longer term if the Germans control most of Africa or the Japanese are able to take the Dutch East Indies.  They would simply become vulnerable outposts that the union would have to fight to defend, very much like Britain was currently.  Also presuming most of Africa comes under Nazi control, as well as the massive loss of resources, this makes Latin America vulnerable to German influence, which may be welcome to some. 

  It could be that the author actually realises this but is worried about the still politically significant isolationist movement in the US.  Hence he's trying to lure them into cooperating with Britain for mutual defence.

  I must admit I had missed that.  Suspect as you say its a compromise between Washington and London. 

  That would be a very explosive issue, especially since there are also white minorities settlements in Rhodesia and British east Africa.

  At this point I think the blacks and coloureds [mixed-race] had more rights in Cape Province, including a limited franchise than effectively the blacks had in much of southern US.  [This was lost when the Boers and right wing English took power in the dominion ~1948].

  To a large degree the vast majority of non-whites in the empire were 2nd class citizens in the empire, although it varied from place to place as to the degree and in areas such as India things were changing while rich or priviledged groups in many areas still had a lot of power.  [Indian princes, local tribal chiefs, sultans in Malasia for instance].

  Again this comes into does the proposed union seek to defend the empire?  If so, for all the claims of some in the US, the status of the native populations  won't change much and could decline in some cases.  Which could be important in helping or hindering that defence.  Alternatively if the bulk of the empire isn't defended then S Africa at least will almost certainly be lost, although the more racist elements could well side with the Nazis.  Also that would mean a huge loss of resources and bases as I mentioned before.

  That might be a danger although hopefully the example set by the Nazis, along with elements in both British and American culture which oppose such ideas and a commitment to democracy and rule of law would prevent that sort of attitude becoming more than a fringe element.

I wonder what would happen to the rest of the British Empire.

Would the African, Asian, and Caribbean colonies eventually get a vote?