To bring large amounts of cash into Thailand I need to declare it with customs at Suvarnabhumi airport. Where, at the airport, do I declare cash? The only place I see a customs person is next to the X-ray machine, after clearing baggage pickup, and next to the exit.

When you enter there you have 2 channels.

 

1 is green (for nothing to declare) and the other one is RED which says something to declare, so you go to the right (RED)

More likely you would have to declare it when leaving your own country, I think the UK is no more than £10K in cash is allowed

 

$20,000 or over into Thailand

 

"There is no limit to the amount of physical currency that may be brought into or taken out of Australia. However, travellers entering and departing Australia must report any currency they are carrying of $10,000 or more in Australian dollars, or the foreign currency equivalent"

I know you have to declare it when leaving Australia. In Melbourne there is a counter just after immigration where you can submit the declaration. You also need to declare amounts of currency when entering Thailand. I assume it is at the red channel, things to declare.