it's the same with football players. e.g. switzerland has many players with an immigration background from albania and former yugoslavia and no matter which national team they play for people always get pissed
yeah, if they play for switzerland than people complain that they aren't "real swiss" while people in the country their families are from complain about them abandoning their roots. and if they don't play for switzerland then people complain that they only came here to benefit from our system and that they aren't integrate and more loyal to a foreign country.
it's basically the sports version of the "unemployed immigrant that also steals your job" meme
Does the player not decide which flag to use himself? He is almost certainly Polish by ethnic origin (judging from his name and look), but if he identifies more as British since he lives/grew up there, then I think he should be allowed to use their flag if he wants.