Currency of weight-based shipping module

I am setting up the weight-based shipping module, and for testing purposes have simply entered 999999:6.00 as my rate. However, shipping is being charged at £3.79, so I presume the shipping module is using USD whereas my store uses GBP. How can I rectify this?

Take a look at this post for common issues people have:

http://forum.opencart.com/viewtopic.php?f=114&t=108317

Currency rates can be edited in System > Localisation > Currencies. Your default currency is set in System > Settings.