The missiles on the UK's nuclear missile submarines were fitted with decoys. One of the three warheads carried by each missile was replaced with a dispenser that would deploy 27 decoys. A single submarine carried 16 missiles, so it could launch 32 warheads and 400+ decoys at Moscow. The Moscow defenses had 100 interceptors, so it was pretty much guaranteed that at least one warhead would make it through.

More recently, they upgraded to newer missiles which could carry 8 warheads each, allowing them to overwhelm Moscow's defenses without decoys.

See: https://en.wikipedia.org/wiki/Chevaline