Penguin in Wiltshire?

Obviously not, but can anyone help identify a bird I saw but couldn't photograph yesterday? 

It did look like a small penguin. 8-10 inches high, very upright, white front like a dickey and dark, possibly black head and back. Too far away to see it's beak/bill. The nearest I could see in the Rspb guide was Auk, but I would have thought that that is almost as unlikely as a penguin in rural Wiltshire.

Thanks