But unfortunately they haven't. The English trait is called the "Sun Never Sets," which is a reference to the British Empire. One of the English unique units is the Ship of the Line, which were predominantly used in the 18th century - after the Act of Union. Also, if you read the England's entry in the manual, there are more references to Britain than England. (The same has been true in previous Civ titles; for example, in Civ4, Queen Victoria and Churchill were
English leaders and the Redcoat was the unique unit.)
One of the (on-topic) arguments in this thread is that Firaxis should choose either England or Britain and try to be consistent. Given that when most people think of England/Britain they think of the British Empire period, and given that - as the OP says - the British Empire was the largest in history, a lot of people are of the opinion that Britain rather than England should be in the game. If instead you think of the Tudor period (which having Elizabeth as leader would imply) then England should be chosen but references to Britain should be dropped (as much as possible).
Of course, most people probably don't care (but then nobody is forcing them to read this thread
).