The Prius is a conservative automobile

I think it is backwards. "Conservative" and "Liberal" are just labels, used in the context of traditional two party politics, and it is mostly a result of what happens naturally when you split the population into two similar sized groups based on their political ideas. One candidate will target "liberals", the other "conservatives". The definition isn't fixed, conservative ideas become liberal and vice versa all the time in order to maintain the roughly half/half two party system. There is no conspiracy here, it is emergent behavior that is seen in most democracies.

There are similar phenomena in marketing, which is not surprising since politics and marketing are essentially the same thing, but it doesn't mean the categories are the same. The Prius is not a "conservative", "woman", "white", "middle-aged" or whatever automobile, though there may be some correlations. In reality it is an automobile for Prius customers, and what Prius marketing thinks their potential customers are.

Why would I buy a truck that still required me to go through someone else to move large loads? Why would I buy vegan beans that required animal harm to be produced? Why would I buy an electric car that ruined the environment? That's not signaling by consumption, that is things serving their intended purpose. It's no more political than advertising toilet bowl cleaner with clean toilets. People consume what they value for the reason they value it. That what people financially value correlates with what people culturally value is hardly surprising. I'm not watching millennial targeted netflix shows alone in my living room to demonstrate to my furniture how much of a millennial I am. The fact we all consume things that others would consider weird or otherwise socially disapprove of only makes sense in a world where we are trying to satisfy internal desires despite the potential social signals they might send.

Bought a Prius five years and it's been a great buy as a transportation appliance. It can get up to 1200 km/42L tank if you're just cruising at 90kph. Nobody here in Western Canada either wants to climb in to it or thinks its a car that a conservative (in the MAGA sense) would want to be found dead in.

I think you're describing a tipping point - where the association is so strong that it inhibits other uses. The association in this case is "a Prius is a minicab" - which seems to be mostly true in London. I'm sure it's a practical vehicle, since the choice of minicab vehicle is necessarily a very practical one, driven by factors such as Return on Investment, carrying capacity, and finding a vehicle that one can comfortably spend double-digit hours per day inside.

But now, we don't want one since it's going to be assumed to be a minicab.

It helps make them go away, over time, in the same way that refusing to use group statistics to make judgements about individuals (at a cost to your own predictive power) helps redress the inequalities that led to differences in groups in the first place.