[pas]

Okay, but what kind of voting system can capture this well (and what does it mean)?

We don't know how people would have voted in this hypothetical. (Likely there would be a lot more parties, which generally is good.)

Also C+R could have formed a coalition. (Or merge into a new party ... or - I haven't looked up how R came to be, but I assume it's a spin-off of/from C - R could have merged back into C, right?)

[throw646577]

Reform is a spinoff, yes, in the same way that UKIP is a spinoff: it's a party consisting of people who didn't/couldn't/wouldn't get selected as conservative MPs, plus on the odd occasion one or two who left (Lee Anderson, who is terrible, awful, self-serving and alarming, and before him for UKIP Douglas Carswell, who was largely better characterised as a nice enough bloke who was completely wrong)

[pjc50]

The Scottish and Welsh assemblies use AMS. The NI assembly uses STV. You can see how this produces completely different results from the Westminster elections held in those areas.

Reform are, like UKIP and the Referendum party, Trumpist parties organized around a popular figure and external funding (Richard Tice). They're not really spin offs although a few MPs may cross over.