Hey! Does the Montana DOR website give you any province/state codes at all? Because the UK should be one of the (few) countries actually covered by that list? Here's the IN.DOR list, which has UK codes on it: https://www.in.gov/dor/files/reference/foreign-state-province-codes-2018.pdf You could try the code listed for your region here?
I found my Saudi province's ISO code (01), noticed that some other countries (Japan, etc.) had ISO codes sans the 0s listed as province codes (Hokkaido's ISO code is 01, for e.g., but its province code for IN taxes is 1), and kept entering "1" as my province code until TurboTax finally relented.
You could try that, too? (Unless the UK requires a spec. letter code instead of a numerical one in Montana, too...)
