For visa exempt entries it’s 60 days. That may or may not change to 30 in the future. There’s talk of it doing so but currently just press speculation.
British passports are handled through a private company called VFS Global. They have offices in Bangkok or Chiang Mai that will handle renewals.
You need to book an appointment with them to apply.
Current turnaround times are about 4-5 weeks. You need to collect it rather than have it posted to you but you can nominate someone to collect it on your behalf.
You don’t send your old passport away. You need colour photocopies of every single page (used or not) to go with the application.
Or contact Marks Sending Machine here on Facebook for a great service that does it all for you.
Best to go to Room 103 at the Phuket Town office and ask for the handout for the latest requirements. Your TM30 needs to be “current”. That means dated later than your last entry to Thailand or dated later than your last stay in a hotel.
The drive thru at Phuket will update it in a few minutes.
You certainly cannot teach in Thailand on that visa. Well legally that is. To teach you need a Non B visa (if not married to a Thai) and a valid work permit.
An Ed visa doesn’t allow you to teach. As the name sort of implies it allows you to study. Which is something you should do before suggesting that people can do illegal activities.