you apply for a 30 day extension at the discretion of immigration. You need your embassy letter and copies of email correspondence with them. Plus residency documents.
I applied for it today. Still waiting. AirAsia cancelled my flight to Singapore so am hoping. Richard Barrow tweeted the UK government website address and link today.
Chiang Rai is easy but far for you. Can get a 15 day temporary passport at Myanmar side, Myanmar immigration keep your passport. Return same day if you wish or stay Myanmar side. Depends on your eligibility for 30 day visa exempt.