Low Flying Planes

I'm less confused now but one thing that has been puzzling me since just after Christmas is that planes are flying much lower than they used to over where I live. There's no obvious reason for this given that i'm about 15 miles (as the crow flies) from Leeds Bradford Airport but they are definitely flying lower since Christmas and it's becoming more and more regular to see them at the same altitude as i'd expect to see the police helicopter. Well I think so anyway, it's quite hard to tell obviously but they're noisy anyway. I took a picture from my drive on full zoom to show a plane flying low. It's not good as there is nothing to compare it to but you can see it's a plane. Can anyone solve my puzzle? Why are planes flying lower???