In matters of immigration and citizenship, and many other things besides, the UK (or at least HMG) seems to make policy on the hoof to suit the latest tabloid headlines or readers 'phone-in!
It's no exageration to say that "Foreigners" helped to save Britain in 1940, aircrew from just about every nation in Europe (and the USA!), were strapped to aeroplanes and sent off to fight before they even learnt the language, (I met some RAF officers in the Sixties who were still struggling with that!).
In our own inconsistent way, we were glad of anybody to fight for us in the Forties,they were then dumped to favour the Commonwealth; glad of Commonwealth citizens in the Fifties and Sixties these dumped to suit the EU; now, in the New Century, were ungreatfully treating EU citizens to pander to US paranoia.
After all this, I don't know why any non native would want to fight for UK (1940 being an obvious exception!), but if some do, then, properly vetted, make them welcome, be greatful and include citizenship with passport if they want it.