My limited understanding of the new FIG regime is that new / newly returned residents to the UK who have been living overseas for the prior 10 (?) years could opt to continue to be taxed in their previous country (in this case the US) for up to 4 years following their return. So there may be an opportunity to continue Roth conversions for a few years and pay the (usually lower) IRS tax rates and get the future benefit of tax-free growth. As I understand it, any growth within a traditional IRA are subject to tax somewhere, either IRS or HMRC. But Roth growth is tax free in both countries. I'm in the process of getting a better understanding of this myself from a UK tax accountant, so I take full responsibility if I've misunderstood things. Tax verbiage is rarely clear and concise in either country.....

Growth within an IRA is never taxed until distributions are made and then all distributions including growth through capital appreciation are taxed as regular income (no cap gains tax). Growth within a Roth is never taxed even when distributions are made after age 59.5.

We are fortunate to have retired at age 55 and immediately started to make IRA to Roth conversions. At the time we started we didn’t have plans to move back and were doing the conversions to avoid higher US tax bands when SS, OAP and required IRA distributions kicked in. (At age 72? you have to begin taking distributions at a % rate set by the IRS). 11 years later we moved to England by which time most of our IRA money was already in Roths which is going to save a bunch of taxes by avoiding the 40% band on distributions. In fact this last 3 years I have been letting my Roth dividends and cap gain distributions accumulate in a money market fund then taking all that out each year tax free. A year ago my wife cashed in her entire Roth tax free, and has bought 2 houses in our neighborhood that she rents out. Her good idea was to diversify in both assets and currencies.

PS
I am not a tax accountant or financial advisor, just passing on what we actually did.

Thanks for this. Always helpful to see what others have done and whether it worked.

I currently have a 401k, but my provider doesn’t support foreign addresses, so I’m planning to move the account to Vanguard.

Some UK residents have complained about declining customer service from Vanguard.
I believe this is linked to them closing down/freezing accounts this year.
AFAIU these are accounts that are not full brokerage accounts, but deal only with mutual funds. I don't know if these accounts have a particular name.
It appears that if you open the full brokerage account before you leave the US, you will not experience the problems.

A separate issue is online access to US online accounts, the problem being companies not sending OTP to overseas phone numbers.
I think this was discussed on this forum recently (and others, as it has been a problem for a number of years).
So a search will give you much more information than I know about solutions for this.
Many people said that registering with Google Voice before they left the States, worked out well.

Great points.

Vanguard brokerage accounts do support overseas phone numbers for OTP text messages when logging on. I don't the terminology but for years Vanguard have been trying to convert users from their old platform to their new brokerage system. I converted my accounts many years ago, it was easy enough to do.

Plenty other financial institutions do not support overseas phone numbers including my US pension provider. I had been using Skype until they went away this year and I tried to get a Google Voice account but as you say you need to acquire a Google Voice number before you leave the USA. I ended up using Talkatone which is fee based and so far has worked well for me.

I set up a Google Voice account while still in the US and used it for messaging just enough to show it as active. At the time I still had my mobile account and number that was used by banks, brokerages etc for verification. After a few months in the UK I ported my mobile number to Google Voice and it seems to have been seamless for my US bank and brokerage. From what I have read, some institutions do not like to use VOIP numbers (like my original assigned Google Voice number) for verification so that's why I subsequently ported. Well, that plus I'd had the number for 20+ years so it's less hassle than giving my US friends a new contact number for me.... All of my brokerage and 401k are with Fidelity and they had no issues with me being overseas. They do restrict my investment options and they screwed up what I needed done for my expatriation, not to mention my 'split year' taxes - but no issues apart from that :-) It was definitely worth several pre-emptive phone calls with banks etc to make sure they could support the move.

Great points

Yes, I would have encountered fewer problems if I'd known this before leaving the US.

There is something else that the OP might consider as a risk-avoiding action, in case Vanguard make further resrictions on overseas customers.
In the discussion about the Vanguard problem, it seemed the best solution was to move funds to the Schwab brokerage.
This could be done without converting to cash and then rebuying the securities. (I've never transferred between brokerages, so don't know how diffiult the procedure is).
Opening a Schwab account, and putting in a small amount in funding, would give you another option in case there are future problems.