Re diversions to UTP
B747-800,
It,s a long way to Phuket from BKK,Surat Thani may be a better option as it is around half the distance from BKK,and the airport is just off route 4.
They don't have scheduled international flights,but charters operate from there time to time,so arranging immigration should be no problem.
They built a new terminal there in the late 90s,so the airport should be a good choice to use.
Other options might be Khon Kean,some 300 Km north,or Phitsanulok,again around 300 Km north of BKK.
Best of luck.