In my recent experience, the domestic segs have posted as C/H, each earning 20 tier points.

Would be very weary if booking the current BA.com birthday promotional fares - the overall through fares are much lower than published T class with dom add-on so I am inclined to think that the domestic segs for these are N class and non point earners. Booking engine certainly suggests this but its not always 100% accurate.

General rule I follow, if the net fare difference between ex LHR/LGW and ex EDI is £126 (or £215 for North American destinations) then the domestics are H. If its anything less, then a call to BA is needed to verify its fare class. Note the published add-on fares do vary between each regional departure point. If you could provide me with more info, I'd be happy to advise further.