What are the visa requirements for traveling from Thailand to Cambodia and back to Thailand?

Right, it sounds like you're confusing the arrival card with a visa. If you're from the UK and you're coming to Thailand for a holiday for 3 weeks, you do not need a visa. You are visa exempt. You will in that card on the plane but it's basically just a bit of procedural form-filling. The immigration officer will rip the card in half, keep the arrival card bit and staple the departure card bit to your passport (doesn't matter if you have filled the departure card or not). When leaving, you fill in the departure card with your flight number and the immigration officer will remove it from your passport and keep it. If you then pass back through Thailand on holiday for less than 30 days after Cambodia you also do not need a visa, just repeat the above. It's as easy as tying your shoelaces!

Gill - where are you from? It may effect things. But from the sounds of things - you get here. Enjoy yourself. Go to Cambodia, enjoy yourself. Go back to Thailand - enjoy yourself. Then go home. Not too much more complicated than that.

I would concern more if you have passport from somewhere not-known-for-tourist or if you have a history of being in Thailand.

The paper in your passport that you fill in on the plan is an arrival/departure card not a visa.

You will get a 30 day visa exempt stamp on each of your 2 arrivals. And as you won’t be in Thailand for more than 30 days on each trip that’s all you need.

You can be asked on arrival for proof of address, onward travel and funds equal to 20,000 Thai baht.

Okay, what do you mean with Thai exit visa and re do another entrance visa? Do you apply for a Visa before you make your way into Thailand or do you plan to arrive without visa?