TikTok services in Pattaya have been providing bank accounts with Bangkok bank for years on tourist visas. 5000 baht. In and out within the hour. I did mine long before I had the DTV
Personally I have never been asked in over 10 years to show a return airline ticket. Worse case scenario is you book a 1000 baht plane ticket to Cambodia whilst at the airport check in desk if they don’t let you on the flight without an onward journey but most don’t care. Most UK airports are all automated check in machines now anyway, not sure where you are flying from