'm wondering, what are the ethics of someone, presumably currently a British citizen, proclaiming his loyalty to another country, with the stated aim of joining that other country's military in the future? How should the British government view this? How should the average British citizen view this?
I don't see this as especially complicated. So long as he doesn't get himself into a position that requires loyalty to the UK (by which I mean him trying to work at MI5, not the basic "loyalty" of obeying our laws, paying his taxes and so on which I do expect and which I do not think is in issue) then what is the issue? Is this really any different to me saying I want to emmigrate to Oz? (Not that I do, awful beer)

This is quite different from saying that he wants to live in the UK but live under Israeli law. Then I would tell him to bog off to Isreal. Which, err, is exactly what he proposes to do. So fair play to him.

What about all of the migrant workwers who work here and send their wages "home"? Should we get upset that they are not spending their money in our economic and thereby supplying good old British jobs? of course not.

This sort of thing is inevitable in a globalised world, and it may well be that the sooner we primarily identify with things other than a nation state the better. Not that England* isn't top, of course.

*ironically a nation but not a state, before some pedant points this out