What Happens When an American Expat Dies?

You need wills in both the UK and US as they are only valid in their own country.

If you die intestate and no close kin is immediately found your estate (if over £5000) goes on a list called the bona vacantia and people have 15 years to make a claim.  Companies will look at the list and attempt to track down heirs and for a fee contend with the paperwork.

Close kin differs depending on English law (1st cousins) or Scots law (2nd cousins).

If no one claims it then the government gets it (the crown) unless you happen to live within the Duchy of Cornwall - then Prince Charles gets it.