Do I need a visa for a layover in Bangkok without changing terminals?

No

No

Whether or not you are changing terminals doesn't matter. People arriving are never in the same area as people departing, and are forced to a certain direction. You cannot just go from one flight to another in the international terminal. You will have to either go through the international transfers desk if you have a single itinerary/ticket you are flying on, or you will have to go through passport control and enter Thailand and then check-in for your next flight if you are flying on different tickets and doing a self transfer.

If the 2nd one is the case, then whether or not you need a visa is completely dependent upon which passport you hold, which you did not include in your post.

Where are you getting confusing info from? Just curious. If British passport you don't need visa for Thailand.

No.