This interesting gull resembles a 3rd winter (4cy) michahellis with the underwing primary pattern showing too much black in p9-10 to belong to a cachinnans and it has a smallish p10 mirror only. But as such it is pretty retarded in many aspects:
1) bare parts colour – at the end of February michahellis usually show bright yellow legs even in 3rd winters/summers and less black on bill and of course pale irises;
2) also greater coverts, secondaries and rectrices are more immature than in the usual early spring 4cy michahellis.
Bill could be heavier to belong to a typical michahellis, but the solidly dark tertials however are quite typical for 3rd winter michahellis and cachinnans. Also typical are the longish wings and the whole structure with longish legs and elongated body which all contradicts the odd argentatus.
The photos illustrate rather dark upper parts being too dark for cachinnans, especially in the picture on the water, where the head and bill shape fits michahellis rather good. The darkish looking small eye could of course remind of cachinnans but it´s not an unknown fact that it can occur on a michahellis at this age.
Naturally one can't tell for sure, but it is most likely a retarded 4cy michahellis. However Poland is not far away and one could wonder if a cachinnans x argentatus couldn't produce such a bird (primary pattern against cachinnans, rest would fit).