When the Birmingham "lion" first walks into frame its unsteady gait looks very much like a large and heavily built domestic dog, possibly a bull mastiff or a mastiff cross; and the colouration of the animal in the video also matches up very well with this powerful and popular breed.
When the animal turns to the right and starts to walk parallel to the observer, its gait does become a little more feline in appearance (leading to all the confusion!) but conveniently for us, at this point in the video the animal's head and longer muzzle becomes less blurred, giving away the creature’s true identity as a Canid rather than a Felid, such as an escaped lion.
Also, the crook in the tail is much more reminiscent of a canine's; though one must admit the crook in a lion's tail is far less pronounced than that of a leopard's or puma's and closer to the one recorded in this video. However, if I had to bet my own money identifying the animal in this blurry footage, I would say it was most likely a large domestic dog, maybe a bull mastiff; as I see many more canine features in the video as opposed to feline ones.
I personally believe there are probably at least three species of non-native "big cats" living wild in the British countryside, in limited, yet viable numbers. However, this video and many other urban reports on file of alleged lion-coloured big cats, are actually nothing more than honest misidentifications of this large and powerful dog breed.