In my experience, when trying to book a J or F redemption with a domestic connection, if there is no J availability on a suitable flight on the domestic leg then BA.com shows as no availability for the whole journey.

Search for redemption availability on the long haul sector to see if their are seats available in J/F. If there is but trying to book the whole journey including the domestic leg shows no availability then work around is to click “Yes” when asked if you want to include a stop over. This then displays Y availability on the domestic leg and allows you to book redemption flights with a combination of Y on the domestic sector and J/F on the long haul sector. It tends to open up far more options when flying from the regions in my experience.