I understand this for the most part but I would like to give an example which I don't understand:
As a US citizen, I buy shares of a UK company. I sell it and make a big capital gain. I pay the UK capitals gain tax because I'm classed as a UK resident.
Surely I would have to report this to the US as well because I'm a citizen, I don't know if the tax treaty makes me exempt because I've already paid the UK tax, or would I end up with double taxation? but the paperwork I would have to do in this scenario would be a nightmare probably.
Consider another scenario. I buy US shares in the US. I pay my capital gains tax to the US and because I'm a UK resident I report it to HMRC. I don't know about double taxation but I'm thinking the paperwork would be less. So investing in the US as a US citizen is less hassle than investing in the country of residence. So can I or can I not open a US bank account as a US citizen from abroad?
A US citizen you are responsible for reporting your worldwide income to the IRS. Assuming that you are taxed in the UK on an arising basis
In scenario 1 you pay the UK capital gains tax and claim a FTC to offset your US tax obligation.
Scenario 2: you resource the capital gains to the UK....you are a UK resident and therefore the UK has primary taxation authority....pay the UK tax and again claim a FTC on your US taxes.
You fill find it difficult to open a US bank or investment account with a non-US address. If you buy any pooled investments (like mutual funds or unit trusts) you must watch out for US PFIC rules and UK reporting funds rules.
Your ownership of a stocks and shares ISA is good UK financial planning, but if you own funds inside there rather than individual shares you will be liable to US PFIC tax and of course you must pay US tax on any ISA gains so the main advantage of the ISA is not available to a US citizen.