Looks a bit like a Goldcrest, but wait for other more experienced eyes to confirm or deny
I agree with PB and Goldcrest; I've taken a still off your video clip and you can see those huge dark round eyes they have and the upside down smile ! Britains smallest bird along with the similar species of Firecrest.
I agree with Goldcrest too. The huge dark eye and plain face is diagnostic, with the small size, of course.