Golden oriole is only a little smaller than a green woodpecker. Problem with Golden Oriole is that it just isn't plausible one would be anywhere near here in winter, or at least not alive. There isn't anything for them to eat here - they eat fruit and insects. Golden Oriole migrate a looooong way. They spend our winters in Africa south of the equator, not just beyond the Sahara but well beyond the Sahara. So if it is here, it is very, very lost. Recent horrible weather over extensive areas of NW Europe make the possibility that there is an alive GO this far north really rather implausible. Especially given the tiny number of birds that come here even in summer. You are more likely to see some North American blown over in a storm than a GO.

A rather yellowish bird that you can see in winter in Britain is the siskin Eurasian Siskin - Wikipedia, the free encyclopedia. It's more finch-sized than blackbird sized. But given that you were willing to accept it might be a yellow wagtail, perhaps you over-estimated its size. The Goldfinch also has patches of bright yellow. European Goldfinch - Wikipedia, the free encyclopedia. It's quite easy to make scale errors with a fleeting glance. I'd say green woodpecker, caught from an unlikely angle, remains the most likely.