Nun - you are kinda of right but:
1. The US returns will need treaty claims, and
2. The UK will tax 90% of US SS, as against the maximum of 85% in the US, so the overall answer (depending on tax rates which historically have been higher in the UK) will be slightly worse if you are in the UK.
After going around the houses an bit I think I need to ammend my original post. As guya points out citizenship and residence are
critical to how you get taxed. So I'll postulate a "rule of thumb" for a UK/US dual citizen, resident in the the UK.
Render unto Ceaser what is Ceaser's.
The country where the income, dividends, pensions etc arise will tax them and then you claim a foreign tax credit
in the other country, this is a result of domestic laws and specifically addressed in Article 1 paragraph 4,
of the Tax Treaty. It seems like common sense that the country where the
money arises would get first bite at the cherry. I've deferred US tax by paying into 401ks, so it just seems right
that the US should get the tax when I eventually take the money out, regardless of where I live. The situation is
probably different if you are not a US citizen.
Some examples might help. Say I earn money in the UK and have interest and dividends, the UK will tax the income
by PAYE and the tax will probably be taken out at source on the investments. As the US requires me to report
worldwide income I'll have to include my UK income on my 1040, but I'll claim foreign earned income allowance
(up to $80k/year) and tax credits for the UK tax paid.
If I take money from a US pension the US will tax me on it as I'm a US citizen and this will appear on my 1040.
As a UK resident I'll have to report my US pension income, but as I've paid US tax on it already I'll get a
tax credit for that in the UK. For the case of ROTH IRAs to the extent that they are tax free in the US ithey
are also tax free in the UK.
A specific exception to this are State pensions from the US or UK which are only taxed by the country where you reside.
Perhaps someone can try to summarize similar situations for other citizenship and residence situations.