Google Maps: Street View: Privacy and Security (UK version)
Individuals and license plates are blurred
We have developed cutting-edge face and license plate blurring technology that is applied to all Street View images. This means that if one of our images contains an identifiable face (for example that of a passer-by on the sidewalk) or an identifiable license plate, our technology will automatically blur it out, meaning that the individual or the vehicle cannot be identified. If our algorithms missed something, you can easily let us know.
I bet that it's a false-positive for a license plate. You could always use the "Report a problem" link at the bottom of the page.
(For what it's worth, it's blurred for me here in the U.S.)
From the Google Maps support forum:
All faces, licenses plates, etc. are blurred to protect privacy. Sometimes false images such as the sign you discovered gets picked up and blurred.
This is an automated process and is not reversible.