Whatever your position on this argument, the legislation that not only allows but forces British companies to offer employment to foreign nationals from Europe and those from Commonwealth countries with direct British ancestry is a matter for Government, not the companies that have to abide by those rules. It is an issue that affects all employers in UK, with the sole (I believe) exception of the Armed Services - who have some pretty odd rules about accptable nationalities themselves.
You can of course lobby BALPA on the issue, but they are powerless to overturn legislation which is fundamental to the UK's membership of the EU - whether you like it or not!
Scroggs