No doubt someone did a 'cost-risk' analysis at some point. Our 'defences' are still primarily poised to detect and intercept foreign high-level bombers coming to drop their load - not low-level light aircraft coming to drop foreigners!
Cost to counter that 'threat' would be huge I'd guess. Suppose some of the 'gucci' anti-ISIS budget could be diverted but you'd need to lever it from the agencies with shiny buildings in London to do that.