He was 70 or so miles out. That means he was headed for international waters (75 miles out). This problem is twofold. The first is that while his vessel may not be a boat, it is a water craft. Water craft have ratings. They have to be rated for specific types of sea travel. Someone mentioned construction and safety and so on. They're correct. The second is that once he hit international waters he'd be subject to maritime law. You can't just head across the Atlantic no harm no foul. You need to have a passport, some kind of port clearance. Insurance.

In addition, think about this. Nobody bats an eye when you're stopped by TSA and questioned before boarding an international flight. Nobody reacts badly to being turned around by border control for not having a proper passport or documentation to legally leave one country and enter another. The Coast Guard acts not just as a defence of our water borders, but as a defacto police force to prevent or mitigate illegal activities on the water. As a result if he were to be reported missing at sea they would be the ones called to perform search and rescue operations. They would retrieve his vessel.

On top of that, his voyage has to potential (even if it's successful and he doesn't get himself killed doing it) to cause an international incident. What if his vessel ended up being taken by strong currents and he ended up in South America or Cuba instead of England? If we're talking straight across from Florida, that's the African Continent. Nobody in government wants to have to explain to another countries government why some random just showed up in a balloon float. The cost of deporting him, the cost of potentially saving him. The cost of having him in custody until he can be deported. That money has to come from somewhere.