Hello

If a Thai arrives in the UK on a settlement visa may they open a ISA account

yes as long as you have a permanent address in the uk.and the relavant visa,my wife had isa's before she had a british passport.

You dont have to be a UK national. FSA rules state 'normally resident' so if she spends more than 183 days in the UK she can legally open an ISA

She will need to get a National Insurance number first to open an ISA and the bank may also want to see address and name ID, passport is ok for name, a utility bill or council tax bill for the address, so its an idea to get her name on some bills.

Andy