I live in Chiang Mai and I've purchased a Thai Airways BKK/LHR ticket.

 

If I then buy a CNX/BKK ticket with the same airline, I presume I am checked all the way through to LHR at CNX and don't need to check in again at BKK and can transit domestic to international airside at Swampy, correct?

 

Thanks

If it were me I would NOT assume that.  From the airlines perspective you have bought two separate flight tickets so I think you may need to collect your cases at BKK and checkin again.

 

I have done this with Thai Airways although I bought the tickets together. I remember being told to go to the International section of the terminal to check in, that way my baggage would go all the way through and I would be in transit. Not sure if that is still correct as the last time I flew with Thai Airways the whole way was a few years ago.

The last time I left I had an Emirates ticket. The CNX to BKK leg was wth Bangkok Aitways. I checked in at the international desk at CNX and my baggage was checked all the way to GLA. Similarly I was issued with Boarding cards all the way. I left CNX through the International Departures and was interepted at BKK by staff who directed me to a bus which took me direct to International Departures.

I suggest you check in at the International Desk at CNX and show both tickets.

I checked it two ways with Thai and they confirmed that check in at CNX will issue two boarding passes and check luggage all the way through, no need to check in again at BKK.