It is possible for it to have been a Wren as colouring does vary and I have seen them with quite dark plumage on the head. Equally a Dunnock may fit the bill. The manner in which they move could also be a clue.
looked at your link female bullfinch possible black head is identical, also has the white band on wings, and shape of bird about right. but the rest of bird was very deep dark brown on wings and body I suppose could have been a youngster it was slow at flying.
john drew not a wren got lots of them here in various shades of brown but tail not upright, also got dunnocks not them either.