Not sure but here's a link to make viewing easier.
It looks like a Kestrel to me.
I don't think its a kestrel it looks bigger more like a Peregrine.
This is a crop taken from your picture.
Thinking back I'd say it was bigger than a kestrel although I'll not rule it out, could it have been an owl? If Id got a look at its face I could probably had a lot more clues to go on :/