I'm not going either to address other issues or to read all the pages and pages of answers by others. The following is, however, simply wrong:
Wills - I always felt I was fine to just have one will. A second one would confuse matters.
The US does not recognize UK wills, and your estate will be treated as though you have no will, even if you have one in the UK. The UK does not take inheritance tax, but the US does. Tip: get a will done up in the UK and have the same will filed with a US attorney. I am in the process of finding out more...whether this can be a UK attorney with US ties, such as being a Public Notary.
Domicile - You can be a UK resident, but chances are you will always have US domicile. And if you have US domicile, you will always be subject to US tax. Even things you pass on to your heirs will be taxed. If you do not want US domicile, you must cut off ALL ties. No bank account, no voting, no citizenship...don't even request to have your ashes scattered in your hometown.
Social Security - I changed my address with the Social Security Administration in the US Embassy in London, and I soon received a report on how much money I'd get upon retirement. The form said I'd get such a report each year.
I didn't. Because I hadn't been filing income tax, I hadn't been receiving the updated reports on future Social Security payments. I have since written to ask they send them to me again. I'll keep you briefed if I hear.
1. A Will valid under English law (2 witnesses, etc.) will be honored in every U.S. state. Even, in most cases, Louisiana which has traditionally had civil-law rules (notary, etc.)
2. If you have two wills, they need to be carefully drafted so the second does not revoke the first. And at probate the probate court will almost certainly want to see both originals -- and unless the lawyers are clever, they may not get them back.
3. One may be best advised to use the California option (common there because probate costs are high): an inter-vivos trust (with, often, the Will pouring over into the trust).
4. Real estate is a particular problem. In England the Land Registry doesn't accept registration in US form or for more than 4 persons. And a "trust for sale" or "land trust" may be misunderstood in American law because it's not a trust at all really.
5. In all US jurisdiction the spouse can (in the absence of a valid contrary pre-nup) elect against the will and get 1/3 to 1/2 the estate regardless of what the Will says.
6. In England but not the USA an estate may be made bankrupt. In the USA insolvent estates are matters of state law. See this case where the English "creditor" happily got nothing:
http://uniset.ca/lloydata/css/harrison/harrison-11.pdf (this was part of the Lloyd's of London scam, and A. Cary Harrison was a victim of the scam)
7. UK "Inheritance Tax" is not an inheritance tax (like, say, the German one or some US state taxes, where the rate varies according to the relationship of the heir) but an estate tax. A Potentially Exempt Transfer (gift) in the UK may or may not be exempt in the US as well.
8. Trust taxation is arcane. Do not have UK-resident trustees for a US trust.
9. Do not include the boilerplate instruction "after my just debts and taxes are paid". Instead, give your executors discretion to pay foreign tax if in their sole discretion it is in the interests of the estate to do so.
10. If you don't understand the above it may not matter: this concerns Americans who own homes in London, and so on. People with money.
11. The English law of domicile has little in common with that of American states and never mid what your lawyers told you: one may have different domiciles for different purposes (as the late great law professor Willis L. M. Reese showed). Here's the case every US law student reads for conflict of laws class:
http://www.uniset.ca/other/css/182NW227.html (Evan Jones died aboard the Lusitania). It is quite easy to lose an American domicile, and one probably does as soon as s/he is naturalized in Britain, if not sooner. On the other hand generations of British people can be "non-doms" as anyone who listened to the Labour Party promises today will know: https://encrypted.google.com/search?ie=UTF-8&q=non-dom+mandelson
12. But wait ... there's more. Think: QDOT and Community Property. The latter is recognized in English law on the basis of immutability (the US states' rule is partial mutability: status of assets depends on marital regime when acquired). A QDOT is virtually impossible in England and totally impossible in Continental Europe. Google it.
13. For Social Security there is a Totalization Agreement. You can find it on the SSA Web site. You can game Social Security (and NICs for a State Pension) to a certain extent, but in the USA the Windfall Elimination Provision will recapture some of your gaming. Still, you'll get Medicare Part A while visiting or living in the USA, something a recent American retiree I met did not -- because she never paid FICA or SET. Make sure you have 40 quarters of coverage: you can contrive to do this if you have a clever tax accountant or tax return preparer. $650 per year, more or less.
