Even if you managed to shoot down a nuke inbound to the UK, it would surely therefore be most likely to detonate at that point which could still be over land causing fallout. Even if it was over the English channel it could still cause global pollution. One does not simply defend a nuke once launched. The idea is you fire one back sadly. That pretty much is the defense. Mutually assured destruction occurs (MAD) as soon as anyone fires one at someone that also has them to fire back, or fancies defending a country that doesn't.
Otherwise there is no defense. The defense is waving your willy around by doing "tests" and making the world know you HAVE nukes, which is why the big countries all have them, so we are at equilibrium. This will forever be the case surely as it is not possible for the world to all make their nukes inoperable at the same time, nor realistic to expect people would actually not be keeping some back.
Nukes complicated everything.