difficult pictures (you know), but I cant see nothing wrong for a Merlin. Surely no Kestrel with that shape.
But then I realized, that I was biased by your question (no offense): I looked at the pictures ones more, isnt the tail too short for a Sparrowhawk, (smaller) falcons? But this cant be judged in your pictures, a foreshortening effect might be the reason for this here.
More: proportions look some kind of extreme, like a cartoon with round body, short wings and short tail. Best fit is indeed a Merlin, but proportions are cartoon-like (I hope thats understandable).
I must admit, that I didnt comment, when I didnt kne, that you, that you suggested a Merlin or a Kestrel (and therefore assuming that you got the impression of a smaller falcon (-like bird).
conclusion? a bird for quiz-modus, but I agree with PW23 best fit is a Merlin
Thank you very much for your help, I knew there are people who can see better than me I should give you more tips for the quiz. Before I took the photo, I saw the bird from the side - I immediately had the impression of a falcon (without any discussion), flight fast, straight, at the same height, frequent wing movements, at a height "above the trees". Seen today, south of Poland.