Welcome to Flyertalk stevebintley, welcome to the BA forum. I hope you will make yourself comfortable here, it's good to see new people join up with us on this forum.

If you want to get your BAEC number on to the SriLankan booking then it should earn TPs and Avios, if that is your preference. The Expedia link is possibly not going to work. At the risk of asking a silly question, have you tried the Manage facility on SriLankan using just the booking reference and your surname? If you can't change it there, you can try it via the equivalent facility on Finnair. Or just call up SriLankan and get them to swap it over. You could try doing so at the airport, but I would not recommend this except as a last resort, it may well not work.

To some degree it depends on what fare bucket you have used, there are some which don't credit much on BA but you would need to check that on the BA calculator. If you don't really want to collect via FlySmiles then you best not have an account with them, it tends to infect bookings and can make life very tricky in the long run.

You earn TPs and Avios on all airlines in oneworld with only a few rare exceptions. Typically you can book via any channel and just deploy your BAEC card as needed when making the booking. I wouldn't advise using Expedia, it would probably be better to go direct to the airline websites: if you hit problems Expedia are not always helpful.