Why is global free movement advocacy considered well within the Overton window on the left side of the UK immigration debate?

Advocating for global free movement, that is to say the dissolution of national borders entirely, is as morally and politically extreme and unconscionable as advocating for outright naked ethno-nationalism. If anything, the former is even more in threatening to the basic principles of British liberalism than the latter, since the latter would create an illiberal and self-contradictory Britain, but the former would literally destroy Britain as a political concept. At minimum they should be understood as the opposite and equally untenable extremes of the debate.

Left, centre-left and even centrist coded communications platforms and groups, including the entirety of the existing political elite as well as the emerging new right populists in the UK recognize this in the case of racist/ethno-nationalist politics. No matter what you think about the secret heart's desire of Farage and his ilk, they are very careful to observe and police that boundary amongst themselves, being caught saying racist or ethno-nationalist things is the main way you get kicked out of Reform, because they want to be a serious parliamentary force. It doesn't matter how popular Tommy Robinson (who also observes this Overton window limitation, by the way, If I'm not mistaken the worst you can find on him as a racist/ethno-nationalist is referring to a taxi driver as a "little paki" among mates while hammered) gets, he won't be welcome in Reform purely on the basis that he is percieved as an ethno-nationalist racist.

So why is there a complete lack of such (even if it is only performative) discipline on the equally or even more demented extreme on the left?