We have an interesting comparison in North America. The major carriers that support UMA are T-Mobile in the USA and Rogers in Canada.
T-Mobile customers can use Wifi from anywhere on the planet for access to their plan. Rogers customers can only use Wifi in Canada. When we travel, international IP addresses are discriminated.
I don't know for sure, but I suspect you will be able to access your voice and data through Wifi. Orange and T-Mobile UK are competitors, unlike the situation in North America.
