When referring to an island can Ko or Koh be used. I was led to believe that one spelling referred to an individual island and the other was for a generic group of islands but that could have been rubbish.

The word for island in Thai is เกาะ and officially it's supposed to be Romanized as ko, but some people write it as koh.

A group of islands is referred to as หมู่เกาะ - moo-ko, just as a group of houses, i.e. a village is หมู่บ้าน moo-baan.

Of course if you romanize it the official way way, people will pronounce it "co" as in Coke.

When in reality it is pronounced more like "gaw" as in gawk. but cut off at the end.

The 'h' seen on some spellings is just part of an older transliteration system used to represent our favorite confounding phoneme, the glottal stop, which pops up at the end of every open syllable containing a short vowel.

As is true with any transliteration of Tai languages, it depends on the developer of the system. It strikes me as strange that ก is transliterated as k, but that is the way most developers from the UK do it.