I think we're pretty much saying the same thing by now. It makes sense if the infection rate of the destination is near 0, or if the infection rate of the source is vastly higher. The smaller the gap the smaller the uselfulness. I don't think it would have helped the UK much at all (save for an unbelievable early introduction with nearly no cases in the UK), the 100thousands who got infected were community transmissions, and to avoid the first transmissions into the country one would have needed a fortune teller. I hate Boris and the tories but the UK had some natural disadvantages, high density population, pub culture, very international, gutted NHS. Their actual failure came in the handling of the sick. But it wouldn't be hugely surprised if there's more people flying between China and the UK than between China and say NZ or AU, despite proximity (or used to, at least).
For a continental European country it makes almost no sense to quarantine with other continental countries at the moment, in my opinion, the costs vastly outweigh the slight benefits even for the country with a lower infection rate. By the same token one could just ask random people to quarantine for the same amount of time, it would have the same positive effect.