Normally you have 7 days before you have to leave. You can leave and re-enter again from another country and if you have the requisite skills you can apply for LTR/smart visa once you are back. It takes 30 days to process.
Do not under any circumstances let them know you have 2 passports. It will.cause immense confusion and indecision. Just tell them the UK authorities asked younto send back the old passport once visa was extended (but in their system its canceled).
Lost my baggage upon arrival, was not given any options for an overnite bag or allowance while they traced it. 2 days later I was told I would be compensated for reasonable costs and given a number but was not able to get through to any Thailand number. I submitted my claim, followed up several times but no response. Eventually my bag turned up and I still haven't received any sort of conpensation