I'm a UK citizen so can't give insight into the US citizen/dual citizen issue (others such as Durhamlad can give good advice on this side of things). I've written below a few things I did, didn't do (and wished I had), or learned after moving to the UK. Some of it may not apply to you, but thought I'd put anything I can think of and you can use/reject as needed.
My situation is that I lived in the US from 1990 to 2006, with a 1-year break in 2000. For some of those year I paid FICA and also paid into two 403(b) plans with TIAA-CREF. On returning to the UK, I quickly started thinking about retirement, i.e., how I would juggle US and UK "plans" when I finally retired (in the UK).
I would definitely endorse the advice to rollover to an IRA, because of the flexibility that gives you. In the past, I've seen reasons given to roll over to Roth, but I think that advice was for reasons that don't apply to everyone.
One hiccup I know many people hit is with the 30% withholding on retirement withdrawals when you live overseas. I've had many tussles with TIAA over this, and although I managed to arrive at a solution, others have failed. Just something to anticipate if you consider withdrawing funds from the UK, as you can end up losing 30% withholding and then UK taxes, meaning you might only net 50% from your withdrawal, until you eventually get a refund from the IRS (which in my case was 18 months after the withdrawal occurred). There's a couple of threads on this.
For reasons purely to do with achieving simplicity, I wasn't keen on the idea of (a) dealing with funds sitting in the US and (b) dealing with issues when withdrawing or receiving benefits from my US 403(b). However, it is nigh on impossible to transfer US retirement funds to UK retirement accounts. I found a belt and braces way around this, and if it's something you want to know, let me know and I'll paste a basic description.