Busking on the South Bank #2

Got responses from both Simon Hughes' office, and Peter John, saying they'd look into my query about Southwark's busking policy. Encouraged by this, I returned to the South Bank on Sunday 12th August for another session. This time, I was stopped by two plainclothes guys with Southwark badges in their wallets. They made a note of my address, and said they were happy for me to recite my poetry, but that it was illegal for me to sell any copies of my poetry pamphlet. So much for private enterprise. Which department do these men work for? Why is it necessary for them to patrol the South Bank in addition to the police, who are heavily present, and the armies of stewards? At what expense to the public finances are they maintained? What is the public mandate for preventing performance artists selling their work on the streets?