My question was to see what the experience of unvaccinated travellers are in terms of treatment by airport/border staff.
If your companion follows the rules and the rules for the country you're visiting do not require vaccination, then I don't see the issue. My experience is: be aware of the rules both for the country (and local area - they might be different) you're visiting and for the return to the UK. Check the official government sources - and keep checking every day, because they can change - and follow them.
If your target country requires a PCR test for everyone, then they will likely check the PCR test results for everyone and will not even know anyone's vaccine status on arrival.
When you go back to the UK, you (fully vaxxed) and your travelling companion (not fully vaxxed) may be treated differently (depends on the country you come from, or where you have been over the past 14 days) as those are the UK rules right now. Assuming your companion is a UK citizen or perm resident, they won't be refused rentry to the UK, but they may have more hoops to jump though than a fully vaxxed person.
While I'm not saying it never happens, it's unlikely that anyone is going to be harrassed by airport or border or airline staff for not being vaxxed in a situation where it is not a requirement to be vaxxed. They're trying to do their job and keep the queue moving, so if vax status is legally/practically irrelevant I really doubt anyone will mention it.