If I were a smuggling drugs, the last thing I'd do would be to do it using a light aircraft.
The UK is such a relatively small place, that anything out of the ordinary is usually noticed by someone somewhere, and invariably Mr Plod gets to know pretty quickly.
Addtionally, the intelligence network in this particular area is a lot better informed than the vast majority of people think or realise, and whilst, due the nature of getting evidence, it may take time to get results, customs and the plod are very successful.
Take a moment to reflect upon whom, at your particular airfield, you know to be a bit of a wide boy or have shady connections, I can think of several people I've encountered over the years who fit this scenario. Well, if you know or think that about them, so will someone else, as invariably, will the police.