Are you saying that those things have disappeared form the Birtish menu?
They mainly certainly have from the diet of the typical British migrant in Poland (due to problems with availability).
as someone whose writings include foodways
You write about food but have never heard of aging steak? Seriously?
to find out to what extent expats retain theri native eating habits
Given that by your own definition you are an expat, can't you answer your own question?
Where have all the expats
They've been replaced by local hires (mainly Poles, but with some foreigners, both migrants like me and immigrants like you).