It seems to be an issue with the way SmartApps are setup for UK users.
Reference Thead: Similar UK IFTTT Authorization Issues
I haven't been able to find much official guidance from SmartThings on this, but from what I've discussed with other SmartThings developers it seems the SmartApp has to specifically be published on the SmartThings UK servers. Many US developers are using an unofficial feature of the OAuth workflow wherein publishing a SmartApp 'for me' will also allow other users to authorize against the OAuth endpoint and have the SmartApp automatically installed for them. Unfortunately, this doesn't seem to work for EU users as they don't see their hub/location when trying to authorize against the US endpoint and if they manually switch to the EU graphi.api.smartthings.com URL, the SmartApp isn't available and they receive server errors.
There was an announcement discussing the impact of region-specific URLs, but the documentation on how this applies to the OAuth workflow seems to be missing as of today. (@Jim ?)