It is easy to forget that defense and security are not the same thing. Defense is what countries must resort to when their security breaks down. And during peacetime, countries spend money on defense precisely because they fear for their security.
Since 2014, the security environment for Britain and the European Union has deteriorated sharply. In March of that year, Russia invaded Ukraine and annexed Crimea. It was the first time since World War II that a major European power sought to redraw its own borders by force of arms.
In 1994, Russia agreed to defend Ukraine's territorial integrity in exchange for Ukraine's handover of the nuclear weapons it had inherited from the Soviet Union. But Russia didn't stop with Crimea; since then, it has waged a low-intensity unconventional war against Ukraine in the country's eastern Donbas region.