First-hand experience here.
In 2010, my Argentine girlfriend and I applied for an unmarried-partner visa to live in the UK — for which we spent five months apart during the application in our respective countries. Upon receiving the news that the visa had been denied to us, we bought a ticket for my girlfriend to fly to the UK to see me on a tourist visa.
Upon arrival in Heathrow she was detained by Border Force and interrogated for two hours before being told she was to be put on the first plane back to Argentina. All this time I was in arrivals waiting for her to appear — she was prohibited from using her phone — the airline nor Heathrow security could tell me where she was. After four hours, a Border Force agent called my mobile and told me what had happened. Two hours after that my girlfriend managed to call me in tears from the bathroom before they put her on the plane back to Argentina.
The next week I had sold up my things in the UK and moved back to Argentina.
After that, when going back to visit my family in the UK as an unmarried couple, we had to travel together and stand in the non-EU-passport queue in Heathrow with a folder full of documentation from her office and University in BA both vouching she had a job and studies to return to, letters from my parents saying they would be accommodating her during her stay in the UK and would be financially responsible for her, return tickets to Argentina, bank slips for proof of funds, and gas bills in both our names, rental contracts etc. to prove that we were a real couple. We were usually delayed and questioned for a while with the Border Force agents disappearing into back rooms with our passports — we had been flagged, so would always be held up from then on — but we always made it through eventually with the extra documentation.
In 2013 we married — nobody, not even those enormous cock-juggling thundercunts in Border Force or the Home Office would rush us — and since then, we proudly waltz through the non-EU line at Heathrow, throw our red/blue passports down in front of the shag-heap passport inspector and high-five each other, making our wedding rings chime in unison.
To be honest, the first passport inspector we came across since we got married was pretty cool. He said he would remove the flag on my wife's passport and went off and did his thing, came back and said after a few weeks we shouldn't be flagged up any more upon arriving to the UK, we now pass straight through with no problems.
TL;DR
So yes, sorry to long-wind it but I am still bitter over the shitty UK immigration policy that has only got worse since we were keel-hauled under it.
Today, we have no problems as a married UK-Argentina couple (her with no EU passport), even after failed residence visa, deportation and being 'flagged' for several years.