The fare calculation depends on how you bought the ticket. If you are UK based and bought using the UK website it should be in Pounds Sterling (GBP). NUC is an IATA virtual currency, designed to make life easier for tickets priced in different currencies, but you can ignore that. But regardless, using the Avios calculator is a good alternative way on the whole, however you would need to remove any seat charges, additional baggage fees, change fees and any other extras. Overall though it seems that you are looking for £1,000 total or thereabouts and so you should bear that in mind. The £200 is ex-gratia in theory, but if BA offers £800 for the fare difference then that's really as far as it goes. I suspect they will offer less, but you need to wait a few weeks until the "back office" has calculated this more precisely.