Defending Britain depends on how it is attacked. Despite recent testing failures the dissuasion threat is credible.
Defending against men on the ground isn’t credible and never was in the last century, even less so now the military is professional.
I am not sure we would have the means to stop them getting here. Your country did in 1940 (and thank God it did) but technology has evolved in such a way that it would happen in a fraction of the time it would have then.
But the worst type of attack against which people on both sides are really putting brain power behind are technological. Protecting power distribution, banking systems, transports and traffic in general, information, elections…
I am not sure we stand a chance. But that doesn’t mean it’s not worth trying. Just not with the current shower.