I believe Lee is part of American equity at this point. If you notice in other shows with trades:
"[Actor] is appearing pursuant of an exchange between American Equity and UK Equity."
There is no such notice in the Mary Poppins program.
Also, I do not believe there is a 6 month maximum on English actors in the US. Maria Friedman was signed on with The Woman in White for 9 months (of course, this wasn't completed).
It's all part of how long the trade off lasts. For example, Friedman was able to sign on for 9 months because Jill Paice, an American, was with the show in London for 11 months. Thusm Friedman's maximum was likely 11 months.
Gavin Creel in London...I have no idea where his trade off is from (unless I am wrong about Lee). He's currently signed through November...that's well over a year by that time for him.
How to properly use its/it's:
Its is the possessive. It's is the contraction for it is...