Xiaomi has been hugely popular with bargain hunters in the UK who have imported the phones from offshore companies such as Banggood / Gearbest / Aliexpress and others.
These phones were originally developed for the Chinese market and normally come with the MIUI China ROM installed. Sellers then often re-install Google frameworks and apps. Users then sometimes flash the China ROM to install the Global ROM themselves so as to get regular updates.
In recent months we have seen Xiaomi expand into global markets targeting India, Indonesia, Spain, Russia and others. These phones will come with the Global ROM and it appears Xiaomi want to restrict people selling the Chinese phones in order to boost the sales of the phones they sell globally.
The information as shared on MIUI forum stated clearly that;
1. Xiaomi Phones manufactured for (the) Chinese market are not able to run MIUI Global ROM.
2. Xiaomi Phones manufactured for Global markets are not able to run MIUI China ROM.
The official statement then recommended that buyers should purchase Xiaomi phones via official or authorized sales channels. In addition, users should double check the system information before flashing or updating. The ultimate result of not verifying could be bricking of the smartphone.
You will still be able to re-install Google frameworks and apps on the Chinese ROM versions but this will need to be done via APKs so you won't get regular updates.
If you don't mind getting a bit more technical you should still be able to bypass this issue by unlocking the bootloader and flashing the device with a new custom ROM. It is normally quite an easy process but it is dependant on you being able to unlock the bootloader.
If you would prefer to avoid these issue then you will need to be more careful about which phones and you buy from where. If the phone has had a global release, to countries like India etc, then you should still be able to pick up Global ROM options.