Fortunately, the vast majority of light ac pilots are decent, honest folk. Because the freedom of movement within Europe, lax laws concerning the drug menace in certain countries and the number of cross-channel flights at weekends means that the unscrupulous might think that it's easy to smuggle illegal substances into the UK in order to make easy money.
But don't forget that Customs and Excise and the Police are not stupid. There is a very good chance that they know exactly who is going where and if a suspicious pattern develops, a few discreet investigations are made; if suspicions are confirmed then a trap is set and 'chummy' is usually caught 'bang to rights' and ends up being detained at HM pleasure. You'd be surprised at the interest taken by C&E - I was ask to pass details of a certain flight from my Club by one of the members as it had aroused suspicion; fortunately we were able to show that there had been a genuine misunderstanding (pilot had announced that he was going to aerodrome X and had set off in a totally different direction; in fact the pilot was going to a totally different and rarely used strip with a very similar sounding name) and we were thanked for our help. Obviously Officialdom had been monitoring local ATC frequencies and was doing a bit of investigating.........a few months later a massive drug seizure took place at a nearby aerodrome as tens of thousands of pounds worth of this filth were taken out of circulation.
So don't think, if you're tempted, that you'll get away with it. Good luck to the folks at Customs and Excise and the Police; most of the scum involved in the drug trade are greedy and will very soon get careless - just before they get caught and subsequently enjoy picking up the soap in a communal prison shower for a few years....