Pterodactyl maybe? (Or maybe a tern)

Hello, I think I’ve just seen a pterodactyl fly over. I’m kidding, but it was a bird I’ve never seen before. I have one photo but I couldn’t focus my camera in time so it’s a blurry mess. Here are all the details, it looked like a TERN to me: 

Size: Small gull sized 

Colour: White/very pale grey underneath 

Shape: Had very slender and thin long wings, smallish body and short tail. From a distance, it looked like a bigger, white swift. 

Flight: Soared in circle for a few seconds but glided away pretty fast. 

It definitely wasn’t a seagull though, thanks Ellis