On the layout where you see this field, enter layout mode.
Select the field by clicking it.
On bottom of the data tab of the inspector is a section for Data formatting. This field is formatted as currency and one of the options available here will enable you to change the $ to £.
Once you've made the change, save your change and return to browse mode. You'll need to make the same change on every layout where you find $ as the currency symbol.
Phil, is there a way to setup an invoice layout where you can choose a currency to use. So instead of going to the fields individually and changing them in inspector, having a drop down list to select a currency symbol to apply to that entire layout in the fields that require a currency symbol?