Congrats on the move!
You don't need to do anything with the IRS. You'll update your info with them on your next tax return. You still have to file US taxes every year, and depending on where you are moving from, state taxes as well. Hopefully you have already set up how your tax situation will work with your employer.
You'll need to set up a UK account as early as possible over here but it can be quite a task to do so. Have a plan for how you will access money for about a month (probably just ATMs). If possible, you may want someone in the US as a signer on your accounts to help you move money over.
You'll also want to be prepared to not have a personal mobile phone for a bit, as you'll need a bank account first.
Pack some comforts from home. My short list includes deodorant (it's all spray here), nice sheets and towels, and ranch dressing.