I don't have a reference, but as far as I am aware, as long as you remain "airside" you don't need to get checked at the airport you tech stop at.
I would imagine that that is only true where there is a clear distinction between airside and landside, with custom/immigration available at the crossing point.
Land in a small, open airport, and I think you've effectively arrived in the country and therefore need customs/immigration.
I don't think you'll easily find much saying that you don't have to clear customs if you stay airside. If you look at the Terrorism Act 2000, and the various customs/immigration acts listed on the UK GAR, you'll see that they say you need to inform or clear (as apporpiate) that agency when arriving in the UK or when leaving.
Of course if you land in Gatwick, and never leave airside, then no one will care. Arrive in some farm strip, and it might be different.