United Kingdom

The UK must "actively prepare" for a potential "wartime scenario" in its homeland, the National Security Strategy has warned.

The review highlighted Russia and Iran as potential threats, including saying that the latter's "hostile activity" on British soil is increasing in an effort to "silence critics" as well as "directly threatening the UK".

If you read the article that they're linking to, it does say that this is a "low probability" scenario.

"We are trying to raise awareness through this war game to say, look, let's have a look at what might happen," he said.

"Unlikely and low probability though it is, so that we can start to put some measures in place and remind ourselves about how we used to do it - use history as our weapon, if you like, in that regard."

Russia has aimed to assassinate people on British soil and has engaged in sabotage in Europe, but that's also a long way away from what I'd call a real wartime situation.

Vice asked a guy at Jane's to do an assessment of whether Russia could do an invasion of the UK a decade back. He said that Russia likely could:

The requirements for carrying out a successful invasion are pretty substantial, which makes the list of realistic threats to Britain quite small. The bigger military powers are an obvious contender to begin with; the USA and Russia have certainly got the manpower and capability to carry it out but China, for example, doesn’t yet have a global reach and couldn’t support enough troops and aircraft that far from home to make it viable without support.

That being said, that was also pre-Russo-Ukraine War, and that may have altered things (China's certainly built up her military a lot over that decade). Might be interesting to go back to Jane's and ask for an updated assessment.

Iran could engage in terrorism, maybe smuggle some weapons in, but doesn't have the force projection capability to engage in much by way of conventional war on the UK territory. Iran's current ballistic missiles don't have the range to reach the UK from Iranian territory, so unless they can launch them from ships or aircraft or closer territory, they aren't in the picture...and while you could probably hurt the UK with said missiles if you could get them close enough, hit seats of government, power plants, I don't believe that it'd be sufficient to take the UK out of the fight, and then you'd be in a fight with an opponent who has more force projection capability than you do who is at a range that you can't easily hit them at; not a favorable situation. Not to mention NATO.