Just to clear a few things up, as no one seems to know and there's different advice online

Bit if a panic as I'd left my passport at home but had a photocopy

I traveled Chiang Mai - Bangkok on Thai Smile, passport copy was totally fine and I called up first to make sure. Flight check in, security and Boarding no problem whatsoever.

Thai drivers license does work as photo ID as well.

Your English language driving licence from back home works as well. All that is required is a government-issued photo ID in English (or at least your name in Roman characters).

I am not sure a passport photocopy will always work, though. That could be possibly altered or fake.

Glad it worked out for you! A drivers license should be fine. I am a bit amazed they accepted a copy of your passport as the only form of ID. Did you also have to show a credit card?

I'd never travel far from home without my passport. Especially to Bangkok where there's been lots of scams by various officials lately. And if something goes wrong and you're involved in a difficult situation, lack of a passport could be very problematic. It's rare, but does happen.

Just another quick note: most hotels will require your passport to check in. They are required to submit passport and TM card information to the police. Some locales enforce this more than other, but you may find yourself unable to check in if you do not have your passport.

Regarding ThaiSmile: I would call accepting a passport photocopy as positive ID to be a significant security lapse. No airline in Europe or North America would ever allow a passenger to board any flight with a photocopied ID. Such security lapses on behalf of ThaiSmile are probably just another reason why ICAO and various countries are issuing unacceptable ratings to Thai-flagged airlines.

I have stayed in a lot of hotels and the Thai DL always works to check in.