There's currently a direct flight from BKK to Savannaket on Lao Airlines, which takes about 90 minutes. It's not cheap and it might be discontinued soon.
You can only cancel an Extension of Stay, not a visa. If you have an Extension based on work, you cancel your WP then the Extension and leave the same day, or get the 7 days to leave mentioned by
She will need a Visitor visa if she's just visiting. She applies via VFS in Bangkok or Chiang Mai. If she plans to live in the UK she needs a Settlement Visa. Easy enough, if you meet the financial requirements and she passes the English language test.