re: "My application auto populated with Vietnam as that is where I am right now however my base country is the UK (town Haverfordwest). Can you fix it?"
There may be nothing to fix.
Upwork's location field is intended to display where you are located.
It means: Where does this freelancer currently live?
It does NOT mean:
What country was this freelancer born in?
Where is this freelancer a citizen of?
What nationality is this freelancer?
So if you live in Vietnam right now, then your Upwork profile location should be VIetnam.