as i’ve said before - do NOT compare Trump to BoJo. Trump was a successful businessman and he kept his promises. BoJodidn’t do that when he was foreigne secretary.
As to the question of why population substitution, the answer is applied cosmetic gene therapies.
How many genes and/or expressions do you think determines eye color, hair color/texture, skin tone, etc? Would you be surprised the number is quite few, and the promise of being able to change (superficial) appearance is poised to become a large part of the personal appearance market?
Now for the larger goal: IQ improvement. Consider nations investing in the modern "moon shot" by being able to move avg population IQ levels 1 std deviation: no more religion (or at least primitive tribal versions). So, like any good breeding program, the existing spoiled, demanding and entitled subjects will be cross-bred and/or replaced with a more determined, less privileged class willing to work hard for little/less pay.
And yet, at the end of the day, they will still be agnostic, have blonde eye/hair, posses even/fair skin tone, and will be peaceful and complacent. If you were an elite, you most assuredly would be in favor of this type of human animal husbandry. And so, that is of course what they are doing.