I noticed this as well when looking at GLA-LGW-NAP-LGW-GLA in CE. Booking via AA.com seems to get the full I class including the domestic sectors (and was also £30 cheaper). From replies I received when I asked about booking on AA.com the downsides of this are no 3x miles if booking with BA Amex PP and no OCLI/SSCI option. However I sat down and worked out that I would receive 3,000 miles (3 people - 2 Domestic Legs - 500 miles) and the 40 TP's each. This worked out more than the 3x miles advantage.