Who you pay should be set up as a supplier

Then if you enter a bill for the supplier, paying more than what is due to the supplier will record a supplier credit that can be applied to the next bill.

I do not know UK accounting regulations, but in the states, your situation would not use COGS, it would use a cogs type account titled COS< Cost of Sales.  COGS, here, when you file taxes only deals with inventory sold.  I would suggest you check with a tax ccountant