This graffiti is on the corner of Clipstone St and Cleveland St in central London. I believe it is by the artist Banksy, and the perspex covering would suggest somebody has decided it is worthy of protection. You have to step up quite close to read the additional message, scratched on the perspex. "Fuck Banksy - Team Robbo 4 Life"
There is a long-running rivalry between the two artists and their supporters, documented on King Robbo's Wikipedia page, here