The UK doesn't have a single written document that lays out the Constitution, but I wouldn't necessarily call the Constitution informal.
> its executive is subject to the legislature in a way it isn't
True.
> one house,
The UK still has a bicameral, not unicameral, legislature, though it now has priority in the lower house (unlike the US, which retains greater power in the undemocratic upper house, a feature it copied from the UK which has since shed it.)
> The UK is a parliamentary democracy and the US is a republic.
The UK is a representative democracy with a ceremonial monarch and the US is a representative democracy without a ceremonial monarch; the absence of a monarch is the sum total of the difference indicated by “republic”.
> Also, the UK wasn't a democracy in any meaningful sense in 1776.
Neither, though, was the US in 1776, or 1789, for much the same reason: the colonies had imported and retained (in some cases added to) the kinds of restrictions on the franchise found in the UK, and kept them past the revolution and Constitution, which left decision of who could vote to the States (and, while not in the federal government, also often had even more stringent property, etc., requirements for office holders.)
The UK doesn't have a single written document that lays out the Constitution, but I wouldn't necessarily call the Constitution informal.
> its executive is subject to the legislature in a way it isn't
True.
> one house,
The UK still has a bicameral, not unicameral, legislature, though it now has priority in the lower house (unlike the US, which retains greater power in the undemocratic upper house, a feature it copied from the UK which has since shed it.)
> The UK is a parliamentary democracy and the US is a republic.
The UK is a representative democracy with a ceremonial monarch and the US is a representative democracy without a ceremonial monarch; the absence of a monarch is the sum total of the difference indicated by “republic”.
> Also, the UK wasn't a democracy in any meaningful sense in 1776.
Neither, though, was the US in 1776, or 1789, for much the same reason: the colonies had imported and retained (in some cases added to) the kinds of restrictions on the franchise found in the UK, and kept them past the revolution and Constitution, which left decision of who could vote to the States (and, while not in the federal government, also often had even more stringent property, etc., requirements for office holders.)