From what I heard recently you must have the local firmware installed to get Samsung Pay to work. So a Korean device as-is won't work for Samsung Pay in the UK but since either place would require the international version, the Exynos processor viersion, of the device you should be possible to flash the UK firmware to the Korean device.
The problem it seems would be if you purchased a US device and wanted to use it abroad or vise versa. There doesn't seem to be international firmware for the Snapdragon processor model, used exclusively in the US, or US firmware for the Exynos processor model, used almost everywhere else.
I'll look and see if I find the posts where this was recently discussed and link them here.