I'm trying to set up some UK address data in my AX7 Azure demo image but for some reason on the "Street" tab I cannot change the country / region from RUS to GBR. Any clues gratefully received.
Then you're at a wrong place. Non-Russian functionality doesn't use any separate table for street names, you simply fill in text fields (Street, StreetNumber) when creating a new address, e.g. when adding customer's primary address.
I don't have any AX 2012 on hand, but I'm 100% sure that LogisticsPostalAddress has identical Street field in AX 2012 as in AX 7. It's an actual text field, not a relation to another table. I'm not sure that you mean by "AX2012 allows me to enter a street code and name". That's not the question, is it? AX 7 allows that as well, your problem was about Country/region. If you meant that you can change Country/region in AX 2012, maybe Microsoft added extra logic to AX 7 to prevent users from unintentionally trying to use the Russian logic for non-Russion addresses. Look at the table name to be sure whether it's the Russian table.
I'm in USMF company, but it's irrelevant, as far as I know. Using a different company won't change anything from what I explained above, such as the table won't suddenly stop being *_RU.