Automatically converting non GBP payments into GBP amounts.

Hi there,
I have some sales records that contain GBP payments, along with AUR, USD and EUR.
The GBP payments are not a problem, but a formula which could automatically convert AUR, USD and EUR into GBP payments would save a lot of time and reduce any chance of errors.
I have attached an example of the current layout.

Ultimately, I would like to put the sale in it's GBP amount where the boxes are blank & then a formula that detects either
A) If Original currency (the F column) is not GBP, then multiply the amount in the C column by whatever the exchange rate is in the G column.
B) If the exchange rate column G, has anything other than N/A, then multiply this by the amount in column C.

So, if the original currency in column F is in GBP, then I can enter data into the C column, and it will not change, however if the original currency in column F is not GBP, I would like the data in column C to change.

I appreciate that this may be a confusing explanantion, please let me know if it is unclear and I will try explain it again.
Thanks in advance.