Ex-Euroland fares are frequently not available from BA in the UK, so it is quite normal to have to book through BA in Clogland or Bratwurstland.

As for combineability, I have also had trouble bookingan ITA fare, which has started in one Euroland and returned in another Euroland. I believe that here the BA system was right, as I couldn't get the ITA price to work on expedia or anywhere else. I think that certain fares from certain Eurolands are combineable with others, but many aren't.