You need to investigate UK / US tax treaty, to see if you can roll over the 401k or IRA's into a UK based fund, and how.
The US accounts will be frozen for non-residents, unless you have one of the very few banks that will allow a non-resident to trade. When I left the US, wells fargo allowed trades at first, and then stopped a few years in, because of the hassle for them of filing and license and such. The result was pressure letters to sell everything to cash (I refused), so the only thing I could do was when ready, call and have a "sell all" order, which triggered a 10% early withdrawal penalty. Doing anything else was extremely difficult.
I was able to eventually roll over IRA to my domestic account, but it was tricky for taxes. I had to pay out in the USA, and then reclaim an offset for foreign taxes paid on my domestic return. Penalties, such as 10% penalty for early withdrawal are not always recognized, and I needed a VERY high income year to claim that much in foreign taxes as unused credits did not carry over to future years.
An alternative is to leave the money (in an account that you can't trade on), until age 55+, then withdraw as monthly pension income or in full.
Note, the 401k was not recognized for the tax treaty, only IRA's, so I had to create an IRA. A roth -- just pull the money and take it with you.
Check out the rules for SS pension, if you have accumulated credits. I think if you have over 40 credits, you will get $ direct from the US in future years, but if you have less, you may not get a direct payout, and it is convoluted if you get any credit depending on your country. If you are close to 40 credits for SS, definitely wait until you have reached that hurdle. So much better.
Also, whatever you do, sell 100% of any losses in a non-retirement account before you leave the USA while you are still filing taxes, because non-residents can not claim capital losses while out of country for tax offset, but any losses from prior years (while resident) can be reactivated in future if you ever become a US resident again.
I can't comment on whether the US citizenship is worth it for you. I do know that the US doesn't like the reverse: High income and asset US citizens who leave / renounce citizenship, then still come back to the US regularly. They think it was a ploy only to avoid taxes to the US, and the border can give a lot of problems for you about US entry, effectively banning you, if they decide that you are a possible tax evader.