Canada x Greylag hybrid?
With this in mind, I decided to look back on my blog to see if there were any other reports of a similar bird that I'd forgotten about. Although I didn't find any reports of this type of hybrid bird from Linacre, I did find this post from London that showed a very similar looking bird that I'd seen in Hyde Park in 2012. See link here for a picture. I also did a quick internet search and came up with many images of hybrids between these two species. See here for the images.
Greylag Geese are an annual visitor to Linacre, usually seen in the first few weeks of April, and often in the company of Canada Geese, so it's not impossible to speculate that this bird could be a hybrid of these two species.