Can a UK citizen arrive on a 30-day exempt visa and extend it for another 30 days in Thailand?

okay, as people correctly pointed out it's your aircarrier that you'll have the problem with flying in to thailand without a visa (entering on a 30 day visa exempt entry) but having proof of onward travel after the 30 days you'll get stamped in for.

They can demand you show onward travel within the days you'll be stamped in for (and a 30 day extension is NOT factored in), OR they can have you sign a waiver stating IF you're denied entry you will pay your own way out.

Contact your air carrier BEFORE you go to check in for the flight to see what they want.

You're not gonna have any problem once you get here

Airlines can make their own policies and if the airline's policy is that you need proof of exit travel to get on the flight, then you will have to proof of exit travel or you are not getting on the flight. Since it is airline policy, management can of course waive that, but if they won't there isn't anything you can do except buy a ticket on the spot.

I would advise getting a ticket showing a flight out of thailand to show the airline at the airport.

I was nearly refused a few years ago, even though i had a visa.

Also you may get asked at the airport in bangkok to show proof of onward travel aswell.

Always best to carry the three proofs.

Yes ...but your airline may question you...as before.

British citizens can indeed get a 30 day extension.