That's one odd heron! My first impression on bill size alone was Goliath heron...but the bill should be black. The possibility of a Goliath x Grey happening let alone turning up in the uk is very slim!? The jizz of the bird with that huge bill is also reminiscent of Marabou Stork which is also a slightly outrageous suggestion.....In short I'm not sure what to make of this one.
Are there any more pictures of this bird/ Its clearly an oddity!