You should have got an extension ay the border they will laugh at you. Also the letter must state unfit to travel. I know this mysrlf after getting run over by a concrete truck and spent 3 months 8n hospital
You are british dont know about Cambodia but if you loose youre passport the british embassy in bkk will only issue you an emergency passport you have to return to uk to get a new 1 note this does not apply to renewal of passport