But the point is that these cars, when tested under the European testing, will not trip the US test defeat conditions - I understand they're quite specific, because they don't need to. So when they are tested, they are almost certainly not in the reduced emission mode.
If they then pass the EU tests, then you could not sue the manufacturer for non-compliance or being 'too dirty'.
If you're complaining that the EU thresholds for certain pollutants are too high, then that's not something that the manufacturer is responsible for, and you'd have to sue either the UK government or the EU, which is a completely different proposition.
What you could sue the manufacturer for is if any of the stated specifications, like the mileage figures or the amount of pollutants emitted differed from their published specifications at the time that the vehicle was purchased, at which point you could in theory sue either under the trade descriptions act or under advertising standards laws, but I think that you'd have to prove that there is a significant difference, because everybody knows that lab condition tests are nothing like driving on the open road.
Of course, if you're really taking a stand against diesel as a whole, then you could lobby to get all diesels banned, but that would have such serious knock-on effects for commercial and public transport, shipping, and even mobile and backup electricity generation that you'd be backing a losing horse. Diesel is so engrained in our way of life that you really cannot get rid of it, at least not without a decades long program.