Why is it it right wing to be opposed to mass immigration on a society changing scale, especially as it has been and is being imposed without any reference to the existing society's opinions?
It rather depends on the
raison d'être behind the 'allowing of mass immigration'.
If the reason for allowing or encouraging immigration is to fulfil labour shortages or provide cheap labour, it could be argued, (and probably has been, even by RWR) that it's a right-wing policy.
If the reason behind allowing immigration (especially in the form of refugees) it could be argued that it's a left-wing humanitarian policy.
There are many labour voters opposed to what's happening, and while it continues you can bet your life people will continue to rail against it.
I've argued before that politics is not necessarily a single continuum line, but more of a circular process.
It's possible that so far left or so far right ends up in the same place, perhaps extreme socialism morphs into fascism.
On the other hand left-wing voters may argue for social handouts, etc, and it's the competition for these social handouts that motivates their xenophobia. Of course, we 'educated' people know that immigrants don't claim, or claim far less, or can't claim, but the perception of the left-wing are fixed and regularly reminded by others of lesser motivation.
Xenophobia, on the other hand is further to the right of what we perceive as 'right-wing' politics in UK.
It's difficult to differentiate between those that are simply against immigration because they claim the country is full up, those that simply think that migration doesn't solve problems but just moves them, and those that are exploiting those arguments to disguise their xenophobic or racist traits.
It's the rest of their demeanour and comments that help to differentiate those that hide their true motivations and 'adopt' a seemingly benign argument against immigration.