Renouncing Citizenship - IRA & 401(k)

If someone were to renounce U.S Citizenship, to then become an NRA, (U.K Citizen only) how would the taxation of their IRA and 401(k) be treated? Would  the DTA mean that the U.S. sourced retirement distribution, (periodic or lump sum) be only taxed in the U.K, (self assessment) or would you need to file a 1040NR(EZ) as well?

Thanks

Great question, I’ll be interested in hearing the replies, and add in the question how would the dividends and cap gains from brokerage accounts be taxed as an NRA.