An immigration lawyer won’t help you with any of this anyway, it’s nothing to do with visas.
1. Depends on the bank. Some will close accounts for non residents, but if you have regular payments going in/out then you should be fine. Mine (Starling) has been happy to keep mine open. You can do all of the things you’ve mentioned from the US though if you needed to.
2. File a P85 with HMRC. And if you’re renting your home you need to inform them you’ll be a non resident landlord. That’s it other than the usual people you’d tell if you were moving anyway i.e council tax, utility companies, DVLA, car insurance etc. It’s also good to let your doctor and dentist know, just so they can free up the space and take another NHS patient.
3. Get yourself an AMEX card if you haven’t got one already. Then they’l let you open up a US one using their country transfer service, and you’ll get an instant credit rating in the US. I got a card with a $15k limit and a credit score of over 700 within my first fortnight in the US thanks to that. Get a US phone line set up before you leave so you have it from the moment you land - I did with Mint Mobile.
Good luck!