The majority of cooked chicken in the majority of supermarkets is from Thailand and has been for at least a decade. Also, many uncooked poultry products are not from the U.K.
I stopped buying it ten years ago. Aldi then advertised all poultry products it stocked were sourced in the U.K.
There are still some British brands available vis supermarkets such as Booths.
Iceland never used to specify the source on many of their products.
Some supermarkets stock a mixture of British/EU/Thai poultry so you have to check every product separately.
It’s a minefield!
Lots to consider - air miles, animal welfare, safety checks, additives etc etc.