exaclty.. immigration will not be a problem unless u actually broke the rules.. u just need to prove it to EK (IF and only IF) they check but then again I dont see them stopping u from boarding the flt..

The airline is responsible that their checked in passengers have a valid visa ( if required ) as they have to pay for the return flight in case the passenger is denied entry into the country and the airlines could and should know that the passenger did not has a valid visa.
For this reason the check-in staff will check you’re passport to see if you have a valid visa for the duration of the stay.
Also the Thai immigration can request to see the onward / return ticket when you try to enter the country. ( unless you have a non-o visa )

If the airline is responsibleSo i guess i would have to prove that i have a onward flight even with a different airline

If i did have a BA ticket that showed that i was Returning to the UK the following week then it would only be a issue with the Check in Staff and not imigration.

I wouldn't worry so much about dealing with check in staff. Plenty of business travelers do this all the time. Say you have a meeting in Dubai and then in London, but you're not going to stop in Dubai on the way back, so you do SIN-DXB on SQ, DXB-LHR on EK, LHR-SIN on SQ. No one cares that your onward journey is with another carrier.

Thank you for your reply Ung1.
Its a interesting topic really and something that i have never thought about until recently.

Correct.. immigration does not care which airline u fly.. they only want to ensure that you have a return ticket within the maximum period of ur allowed stay in that country..

In the OP's case, the query is whether the airline will have a problem with a nested ticket..OP is not breaking the immigration rules or overstaying so for immigration purposes even if he shows BA return ticket, its fine..

I do not think the above is true. The moment you check-in, your return date is flagged on the screen and the terminal operators are trained to ensure passengers passport or visa is validated against those (irrespective of whether its london or BKK)
If you are checking in with a Thai passport, they probably do not care.
And the fact that there isnt a great influx of brits migrating into Thailand in view of overstaying, this is probably one area the airlines and immigration officials (at BKK) might keep a blind-eye.

Even if you wave your BA ticket from BKK at the counter, thats is not good enough - it is just piece of a paper. Your entry is still controlled at the port by a ticket and passport, as long as your not a citizen where you are standing.

What I am saying is EK may ask that the return date as per their system is after 5 months as per the PNR with them.. Pax can show them the other BA ticket with earlier retn date and it should be fine IMO.. Every e-ticket is just a piece of paper and if that needs to be verified, the check-in person or someone else can easily validate that with BA / online .. Immigration IMO does not care how u return , they only care when and that is proved by the ticket they will want to see ( sometimes only)..