Even a feller like Jefferson, who's quite famous for being a constitutionalist, isn't very famous for being a president.

I assume you mean that he's not well known as a US president in the UK. His administration was very important in the history of the US. It was the first regime change (he was a Democratic Republican while the two former presidents, Washington and Adams, were Federalists), and it also cemented the power of the Federal government with the purchase of Louisiana from the French, which is particularly interesting because Jefferson and the D/Rs were opposed to Federal power; they wanted it left with the states.