UK Border Force don't do themselves any favours when it comes to 'regulating' who lands via a 'small' airfield. A form (General Aviation Report or GAR) is sent to them in advance and if there's no-one on it who is 'suspicious', they either don't attend or (if the passenger is non EU) fax a visa to the airfield for someone to hand to the passenger. But what if the airfield has no fax machine or a 'responsible' person to had over the visa? And what if there is a 'late' additional passenger not included on the original GAR?
Many licensed airfields are totally unmanned after closing time in summer well before it gets dark so it would be easy to land, disgorge a few people, and depart again.