For an interim followup. I called EI again today to book and purchase my seats on this itinerary. The agent I spoke with examined my routing and expressed her opinion that I would be allowed a carry on as well as a personal item as BA's flights were the longest segment and therefore their baggage allowances superseded that of the shorter route legs. However, she thought that the 10 kg limit would still be in place. Go figure, we'll find out tomorrow.