VFS Global is where she'll have to go for biometrics, Bangkok or Chiang Mai, documents can be submitted or downloaded, the UK government site for visitor visa will transfer you to VFS site, that was 3 yrs ago when my wife visited UK, I supplied 6 months bank statements, I've been here when we applied for 3 separate visits.https://visas-immigration.service.gov.uk/vac-information-page/THA