We have revised our wills recently and went through similar considerations, but unlike yourself, I have no investments in the US. I do have beneficiaries in both the US and the UK.
The beneficiaries in the UK are smart people (barrister, "city" financier, physicist). Even so, I would be very reluctant to dump any US tax or management requirements on them. Whilst capable of eventually getting their heads around the US system if it were required, which it's not, I would be reluctant to force them into a situation of spending the time and money to resolve it. Cash is the easiest solution. As it stands now, there will be UK inheritance tax due to HMRC on the estate, but not knowing what end of life costs may be with no children to look after us means being reluctant to divest too much cash beforehand.
For those of us resident in the UK, there will also be the problem of how the final year US tax return is completed. A job for the executors. Also smart people (biologist, bank manager) but no matter how much is prepared ahead of time regards where to find figures, what to file, and where to find help, it is easy to imagine that final year return not being filed. There's also the inheritance tax situation for the US which has all sorts of possible complications and again, it's possible it may not go any further than sending funds to the US beneficiaries. Even with advanced IHT planning, most in the UK simply don't understand the US concept of taxation which means consulting with a professional US/UK advisor, although the costs for that greatly reduces the UK inheritance tax as well as the eventual inheritance amounts. If there are complications for the US beneficiaries, at least they understand the situation and are there to handle it.