When I first started studying this subject, I thought as did you that Manasseh was the US and GB was Ephraim.  But as years went by I began to understand last days Ephraim.  The many prophecies of her 7 year curse and utter destruction by fire clearly fit the US.  She is more prominent in the very last days in world affairs as is pointed out in scripture.  Ephraim is said to rule over the men of Jerusalem (Is 28:14) and is heavily involved with the false peace treaty and covenant with death and hell (Is 28:15 & 18).  We are at the center of dividing Jerusalem and the lands of Judah (Israel).  You can even see this in Jer-usa-lem, dividing the city.  The 3 signs in Ephraim of the bush, the 110 and the 400 year blessing point to the same conclusion.
 
1 Chron 5:1-2 tells us the genealogy was not to be reckoned after the sons of Joseph.  Both scripture and history indicates they live together until they split apart beginning with the discovery of the New World.  We are really the same peoples, though the US (also CAN, AUS, NZ, S.Africa) are made up of those who generally were more poor and persecuted.  You can see a coat of many colors in these nations. That the genealogy was to end with the sons, I think indicates they intermixed into one set of peoples.  The names of Ephraim and Manasseh are used to indicate different nations born (founded) by them.  Ephraim is the younger nation.  Biblical references to the "young lions" I believe refers to all these nations.
 
They still appear as allies both in the world today and in scripture.  Deut 33:17, says they push the peoples of the world together.  Actually, I would conclude this has been happening since WW2 as we both came out of this world destruction as predominant powers.  France to a lessor extent.  But, clearly the USA emerged as the greater power.  As we look at Deut 33:16 we can see the people of goodwill who dwell in the bush.  This reflects the parable of the good seed and tares.  Then we can see Joseph lived separate and in Gen 49:26 we can see he was to receive a crown on his head because he lived separate.  This is speaking to the rapture of the bride.  Verse 17 is then speaking to events after this time.  You will note HW Bush first publicly announced the New World Order after Desert Storm.  He was speaking to the forming of the one world global government.
 
GB is definitely a part of this effort and have been steady allies since WW2 up to and including Desert Storm and the invasion of Iraq.  Micah 5:5-6 is Desert Storm.  The 7 shepherds are those allies of the coalition of the lost tribes of Israel (western Europe).  The 8 principal men are those allies of the coalition of other nations.  Kuwait is in the southern boundary of the promised land.  The Assyrian who invaded our land was Saddam Hussein.  He was born in northern Iraq near Mosul, ancient Ninevah, the capital of Assyria.  We destroyed his army on the banks of the Euphrates, the boundary of Shinar, the land of Babylon whose founder was Nimrod.    Desert Storm was HW Bush's war and it was right after this he announced the New World Order.  The invasion of Iraq was GW Bush's war.  You will see in the rest of the chapter that Joseph is a power among the gentiles and then is suddenly destroyed.
 
The multitude of nations mentioned in Gen 48:19, I think points to Ephraim and her united states.  Originally and by Constitution these states were independent, but yet united.  The power rested in the individual states.  This is much harder to see today, even since the Civil War, as those with money and power pushed for a stronger and stronger federal government which they could more easily control.
 
I am aware of the great British empire upon which the sun never set.  There is not much left to it today.  There are the "young lions" which still give some allegiance to the queen as Canada, Australia, New Zealand, South Africa etc.  But, these are separate nations from Great Britain.  The throne of England has no power of rule.  That rest with the PM and parliament. The young lions govern themselves.
 
I think Gen 48:19 reflects the diminishing of Manasseh and rise of Ephraim.  It says Ephraim shall be greater, implying that he overtakes Manasseh in greatness.
 
Abraham, Isaac and Jacob were promised to be the father of many nations.  I think this applies both physically and spiritually.  We have the US with states and territories and GB with Ireland and Scotland.  It too has some far flung territories.  We have the young lions mentioned above.  The rest of the lost tribes form the predominance of western Europe and some of them have territories.  Then of course there is Judah of nation state Israel, the tiny nation which causes the world to tremble.  All these nations are blessed with industrial, financial and agricultural strength.
 
When it comes to the destructions which fall on Ephraim in the curse period, I do not know if Manasseh is included and if it is exactly how.  I suspect Canada will receive of Ephraim's end destruction as a member of the forming NAU.  The NAU will be one of the kingdoms of the 10 kings (horns).  I believe the 3 kings which are subdued (made to fall) by the Second Beast in Daniel 7: 20 & 24 are associated with the NAU.  The end of the 7 year curse is at the same time the Second Beast comes to power.  It is he who kills the two witnesses and subdues the 3 kings.  I tend to believe the first beast (antichrist) will come out of the throne of Manasseh.  He has Davidic blood and much about the coat of arms and heraldry give clues.
 
Steve