... deterrent from who? Ze Germans?
Honestly, what nation state in this day and age do you think would even consider using nuclear weapons against the UK?
I'll tell you. North Korea. But they'd have to FedEx it over first, and we'd stop them at Customs.
Iran has the will to use them, but only against Israel. (Which is terrifying, don't get me wrong, but not a threat to the security of this country.)
No country would risk the international fallout (no pun intended) in launching a nuclear strike against another. The only people that would, are terrorists, and they're not deterred by a country having nukes anyway. Do you think that if a bunch of religious fundamentalists from Fuckmenistan were to attack the UK, that we'd retaliate against the citizens of their country with nukes?
No. We would not.