I am looking to book a flight from the us through lhr to bkk. My question is: I can book the same flight either with the airlines or through other travel sites and they use different numbers but is all the same flight. Is there any drawback from booking using one way or the other? Thankyou

The different flight numbers you may see are possibly due to code sharing by different airlines under the frequent flyer programs. The host airline flight number will be same, and the code sharing airlines will attach their flight numbers as well. Maybe this is what you are seeing?

The different flight numbers you may see are possibly due to code sharing by different airlines under the frequent flyer programs. The host airline flight number will be same, and the code sharing airlines will attach their flight numbers as well. Maybe this is what you are seeing?

Agreed.

And there might also be different numbers used for the price as well

My choice is always price. If they code-share, the service is the same. Why pay more?

If you book the whole trip US-LHR-BKK on a single ticket then the airline(s) is responsible for looking after you if something goes wrong - say you missed your connection at LHR due to a late incoming flight. If you are on separate tickets then in theory you are on your own, although the airlines may take pity on you. The point is though that they don't have to. You may find it comes down to a trade-off decision between risk and cost.

My choice is always price. If they code-share, the service is the same. Why pay more?

Different frequent flier credits.