I think it was your sentence "I had one night in Belfast, then the next day was back to the UK - via the Belfast - Birkenhead ferry" that camperdown9 was picking up on - as you can't go 'back to the UK' if you're already in it. 'Back to GB' would have worked (though perhaps might not be that well understood by some of this forum's international readership) - else 'back to Merseyside' (which then annoys the Birkenheadites who don't like being bundled together with rest of the 'greater Liverpool' conurbation!).

When dealing with NI, I always try and use the most neutral terminology possible, but given there's so much that can be taken to be laden with meaning, that can be hard work! (e.g. 'back to the mainland' / 'back to Britain' both arguably carrying opposing connotations...)