@Abigail **********
it's obvious that you USED/showed an originating flight departing from the UK to Thailand when you applied for the Evisa online.
As you are now in Thailand... and seeking to be able to activate the non b visa by flying into Thailand from a SE Asian country airport....we wish you success in your Entry into Thailand.
I believe that your plan will work... even if entering Thailand from a border run to a land border from Samui.
You got Lucky getting the Evisa online... hopeful your luck continues with your planned entry
Safe travels.