I contract in Denmark which obviously isn’t the same but on my experience I would get accountants sorted for both countries and make sure everything to do with residency/visa/work insurance/tax/healthcare is all crystal clear and sorted up front before you start, your UK accountant is likely going to say they’ll have absolutely nothing to do with advising on US tax. Also see what is actually left of that juicy day rate by the time you’ve paid all the middle men and bills and taxes, it’s amazing how much evaporates before hitting your bank account.
A friend of mine started working in the US and due to bad advice/admin error on visas was arrested on returning from UK, officially branded an illegal alien, deported and banned from entering the US for five years or something. He laughs about it now though.
Your carbon footprint is going to be a shocker, makes me feel better about mine!