A MMPb book I read a long time ago, library has weeded it, and records have been purged. Was card-catalog days too.
Medium bad story, so wasn't super popular, and probably tossed by the wayside by most people.
Setup was that humanity was divided into 4(?) major blocs politically. And described as Uncle Sugar (US roughly), Uncle Vinegar (USSR), Uncle [something] (China) and the UK had been left out in the cold, and formed a default left-over bloc, adding in places from the commonwealth like (maybe) India, and several other small 3rd world countries.
The blocs had divided up the Solar system, with the major blocs getting the nice planets like Mars and Venus, UK bloc ended up with Pluto (and maybe some other places)...
Lots of cold-war stuff, and some hot shooting off-and-on. So the blocs are tense and heavily armed.
Then the aliens show up...
And they're here to conquer, and add us to their empire. They're used to walking all over new races, and not meeting races who've developed war technology. They use lasers, and don't have armor. The humans use slug-throwers and have armor. They come in off the ecliptic (makes no sense, I know), and 'obviously' hit Pluto first. The UK base on Pluto ends up shooting them up, and get their hands on FTL travel + tech, and 'defeat' the invasion force.
Sadly, when the aliens can't walk in over a race, they follow up with a Nova-bomb, which will cause the sun to go nova, and wipe out the problem race's solar system.
The UK learns about this, and since the Nova takes awhile to get rolling (4 months? 45 days?), so there's time to prep. The UK starts building asteroid ships, and exports the bulk of their population. The other blocs think it's a trick, and ignore and pooh-pooh them. Except for China which makes like one harem-ship, and puts their chairman on board...
They manage to flee, and the Sol system is toast, and with it, the other blocs.
The UK outfits one asteroid with tons of depleted uranium railguns and nuclear weapons, and manages to backtrack the aliens to their homeworld. Then they go in for vengeance. Outfitted with mainly Scotsmen, it's very rah-rah, the Scots are badass. They go in tear up the alien fleet, and carpet-nuke the alien's planet. That asteroid is a kamikaze, and I don't believe anyone who was on that attack run lived.