Not true: You can get COMMISSION FREE/PENALTY FREE currency simply by banking with Bank of America. BOA has a deal with Barclays. If you use your US, Bank of America card at any UK Barclays card, you can withdraw at the current exchange rate. No penalties, no commission and no currency exchange fees. You can withdraw about 500 pounds a day using this scheme, so if you need to move a lot of money over, it might take you a few days & trips to complete. BOA also has deals with other European banks including DeutschBank in Germany and Paribas in France, so you can withdraw your US funds in Euros without charge.
Also, the UK Post Office does not charge commission and have one of the better exchange rates around. And if you have a Eurostar frequent traveler card, you can exchange money without commission at Waterloo station.
Whatever you do, don't use those tourist trap "Commission Free" centers in West London. They are commission free, but you ge ta horrible exchange rate.