"Same as if someone somehow got on a plane to uk our border agency wouldn't let them in "
You haven't quite got the hang of this, zoe. When somebody arrives at a major airport, having managed to get on to an aircraft bound for the UK with the intention of claiming asylum or settling here illegally, they chuck their papers away (usually on the aircraft but sometimes after they have disembarked) and arrive at immigration control "sans papiers". Immigration officials may refuse to grant them entry but cannot "send them back" because they have no idea where they came from. They usually end up claiming asylum, being released pending their hearing (as mushroom says, usually with a map and a train ticket to get to Lunar House) and, surprisingly, disappear off the face of the earth.
Immigration control should be placed at individual gates so that anyone arriving in such a way can be returned to the country they last left before arriving in the UK. It would require more immigration officials and my suggestion is that we retrain some members of HM forces who have been farting around getting their backsides blown off in Afghanistan for the last decade or so.