Nun, Thanks so much for the link. Plenty of information there for me to get stuck into and understand the whole situation better.
I was born here, husband in the EEC, but we have lived in the US for 18 years and are also USCs. We have done nothing yet to suggest that our intention is to put down roots in the UK, and want to understand in depth what that would mean for us financially before we make any decisions. DH is currently seconded here with his company. We'll be here for 2-3 years, and then return to the US. We are simply thinking ahead to retirement which will be in about 8 years time.
If you were born in the UK and most of your family were also born in the UK then you are probably UK domiciled and you sound as is you are UK resident, and will be ordinarily resident if there is evidence of your intent to to stay longer than 3 years.
As USCs you will always be taxed on your worldwide income, your residency status in the UK will determine whether you are also taxed on your worldwide income by the UK. If you are not ordinarily resident you can claim the remittance basis for UK taxation so they'll be no tax on your US mutual funds as long as the money stays in the US.
If you decide to retire to the UK you will obviously become UK ordinarily resident and will have to pay UK tax on your worldwide income. Here are some rules to follow if your are a US citizen retiring to the UK with US retirement and mutual fund accounts.
1) Your retirement accounts are well covered by the US/UK treaty. Growth is free of tax in US and UK, but income from them will be taxed by both the UK and the US, but you can use tax paid in one country as a credit towards tax due in the other.
2) As a USC it's a bad idea to own foreign pooled investments like UK stocks and shares ISAs, tracker funds etc. So your foreign taxable investment options are really limited to individual stocks and bonds, savings accounts, real estate, precious metals and expensive and complicated things involving insurance wrappers. HMRC will also tax gains from foreign mutual funds as income if they are not "UK reporting" funds. So the path of least resistance for the USC is to invest in US based mutual funds that have UK reporting status. Vanguard ETFs are a good for this.
3) The IRS does not treat UK based pension funds as simply and generally as HMRC does US funds. If you have a UK pension that you pay more into than your employer then your tax will get a bit complicated because tax professionals argue about it's status under US law. Some take a straight forward application of the Treaty and say that it covers such pensions.....others say that they are are foreign grantor trusts, which opens up a complex tax situation.