Only a guess, but it would appear it was an attempt to navigate around UK inheritance tax rules by establishing the right of taxation of the estate to be under, perhaps, more favourable Pennsylvania inheritance tax rules, with the goal of a lower amount of taxation than under UK rules. One definition of Domicile is the desire on the part of the individual, in spite of an indefinite number of years of living abroad ("temporarily residing in the UK"), to ultimately return to Pennsylvania. Given the comments in the OP, it would not be an enviable task for the executor of the estate arguing 'Domicile' with HMRC.