Hi Patt--
This may very well be legit... BUT there have been stories on the news here in recent months about internet scams involving bogus Cashier's Checks. I would recommend taking the Cashier's Check to the bank and making sure it clears (that the bank receives payment) before releasing the paintings... or at the very least talking to your bank's customer service department about the Cashier's Check system and how to make sure that you receive appropriate payment for these paintings. Cashier's Checks are not cash, and they are not as good as cash. I don't remember the specifics of the scam, but it seems to me that there were fake cashier's check involved and the victims were unaware of the scam even when they took the checks to the bank, rather they were notified that the funds to reimburse for the fake checks were being withdrawn from their accounts (or were owed to the bank) after the fact when the bank was unable to collect from false cashier's checks that were good enough to fool their tellers. Just a word of caution! The scams often involve a 'Shipping agent' such as you describe. BE WARY!
Here's a couple of links to stories about similar Cashier check scams (scroll down on the second one to read about the scam and examples)... though it 's not exactly what I saw on the news... If you do a google search on 'Cashier Check scam' tons of items come up. . .
Overview of Scam (Nigeria is commonly used, but UK is as well)
More about Cashier's Check scams (with examples)
I'd be very leery of this 'customer'.