It'll be whatever is normally the best airline, people celebrating a mythical event will make no difference, but if you're asking what is the cheapest then the internet is your friend.
it applies to UK and ROI citizens not anyone else, someone arriving in Dublin can be refused entry if they don't have the correct paperwork to enter the UK.
Bringing corpses or human ash into Thailand does not require submitting an Import Declaration Form or Special Declaration Form. The Customs procedure involved is listed below:
Importers present the following documents to Customs officers:
The passport of a deceased person
Air Waybill, original or photocopy
The death certificate issued by the Royal Thai Embassy or Consulate in the country in which the person passed away
Other related documents such as a letter confirming death issued by a hospital and a death certificate issued by a local authority in which the person passed away
Customs officers release the corpses and human ash (and keep record of the release) with the importer signing in the pick-up document.