UK SS is fully taxed by the US for a USC living in the UK.
I am wrong about this. I just got confirmation that UK SS is taxed only in the UK and does not even need to be listed on the US tax return. US SS is reported to the IRS on a SSA-1099 so on the US return it has to reported and then reversed in other income with the Tax Treaty referenced. (I start UK SS in February so was making inquiries)
When I was working in Baku, Vanguard wouldn't let me do something (sorry, I forgot what). I discovered that if I declared myself to be retired (but I wasn't) they allowed me. This was about 5 years ago.
I have only ever been retired while living overseas so can’t think of what the difference might be if you are working, but my son is living and working here and has his IRA and Roth IRA with Vanguard. He has been doing IRA to Roth conversions but not much else.