It matters what passport you hold not where you travel from, but let’s say you hold a British passport then you don’t need a visa for a 30 day stay. And no, you can’t get a visa on arrival if you have a British passport.
It’s a numbers game, before my wife came to the UK on a settlement visa we visited the UK and she had visitor visas. Sometimes they’d get refused and we would re-submit without any changes and they would be granted. Don’t be despondent if the first is refused.
