It would appear from a brief experimentation that BA's cheapest domestic "Basic" fares (hand luggage only) are not available through the scheme. 10% off the "Plus" fares brings them roughly down to the "Basic" level so the discount effectively gets you a free piece of checked luggage on domestic trips. It does seem to give 10% off international economy fares compared with the regular BA website, but I was quickly able to find cheaper deals through Skyscanner. Nice to see BA thinking of us though!