The pilot is an idiot because he did an illegal flight and then allowed the BBC to brag about it.
The BBC reporter is on record there saying they paid for his fuel. That is almost certainly a breach of the PPL cost sharing rules. Even if the reporter was the only passenger, the fuel must have come to more than 50% of the direct costs.
The story itself is bollox because you can enter or leave the UK in a
boat, with no checks
The impact on immigration is orders of magnitude below "negligible". This is how light aviation has been for decades.
The terrorism threat is nonsense because the only way to stop the theoretical risk is to ban all aviation.