There's one or two IFs in that! It is true that, at common law, nobody needs a passport. If issued, it is merely a request, by "Her Brittanic Majesty's Secretary of State" to allow the bearer to "pass freely without let or hindrance..." , a request made to foreign governments.
Even taking your boat out across the Channel to another country is not without its problems; you are liable to have it searched by our Customs, and/or asked what goods you have on board,on return, quite apart from what the foreign authorities may require of you when you arrive there. You are likely to be requested proof of citizenship of a country, lest you are a person who is not allowed to land or enter viz. a passport.
At Heathrow, you'd have to try to pass in without inspection, which is pretty difficult, and it may be (almost certainly is;I haven't researched it ) an offence to do so.