If you look at any of the business saving accounts there are offering 0.5% interest on these balances (just took this information from google). So in OC case if they moved the £6.5million they have from pre-orders into there business savings account every month they would be earning £32,500 AER of interest every month they hold the funds.
Small correction there, that would be £32,500 a year, not per month, and most if not all accounts paying anything close to that interest rate have a cap, typically £2m on the account.
Uncapped accounts are typically paying 0.005%.
As others have mentioned, the most utility they get comes from offsetting any overdraft facilities they may have but even that is minor compared to the advantage they get when the supply chain actually gets moving and large volumes of stock arrive to fill the huge number of prepaid orders they have...