"I am however surprised that few UK tour operators are promoting the island of Zanzibar as of yet."
A close friend's family own property in Zanzibar and we've talked about this several times. Looking at her photos there's not a lot of really good accommodation there - certainly not a lot of what the average UK holiday maker expects if they go to the Caribbean or Seychelles for example. It's still a bit of a "traveller" rather than a "tourist" destination. And so the numbers are relatively low