It’s four days before Apple’s iPhone goes on sale in the UK, and it looks like mobile operator O2 may not be prepared for the demand. The operator, which has an exclusive contract to sell the gadgets, started taking pre-orders this morning on their web site to existing customers who wanted to upgrade to the phone. But the Register reports, that the O2 site was quickly “overwhelmed”, flashing an error message and directing customers to call in by telephone. Apparently, O2 was dealing with some 2,000 angry customers who couldn’t get through, with the operator admitting they hadn’t anticipated demand. At least they weren’t sitting on the sidewalk in front of the Apple (NSDQ: AAPL) store forming a line—as some hardcore fans are already doing in NYC.