Re: Why anyway ?
My partner is non EU with UK vist Visa. Im UK, living in Europe.
Drove back to the UK for Christmas a couple of years back, first time using her visa so no idea what to expect. Was surprisingly simples... French exit policeman looks at passports, gives her an exit stamp. Drive on to the the British control, he looks at the passports, no questions, no stamp, wishes us a nice trip.
Leaving the UK, nobody at all in the UK border post at Dover, the following French point was occupied, so I stopped there, they waved us on, I shouted non-EU to the dude who wasn't the least bit interested and still insisted on waving us, so we proceeded.
Entered the UK again via Stansted a couple of weeks before the Visa expired. Immigration dude asks why we've changed plans, to which we look rather confused. So he makes it a bit clearer and asks why we haven't used the visa until it's about to expire. Turns out partner was supposed to get a stamp when she entered the UK last time, but immigration officer failed to do that and who were we to tell him how to do his job :)
So in her passport she has proof she left France in December, and the next stamp was leaving again the EU in January, having magically re-entered without any proof.
Last Christmas they stopped us at both the UK & French posts in Dover so she got her French entrance stamp, still no UK exit stamp...