For US citizens or green card holders interest, dividends and capital gains on ISA investments are:
1. Taxable in the United States
2. Reportable on Form TDF 90-22.1
A Cash ISA will generate interest income, which is taxable in the US and reportable on the annual US tax return.
A Stocks & Shares ISA could generate interest, dividends, or capital gains; each of which would be taxable to US citizens or green card holders.
If the Stocks & Shares ISA is invested in UK unit trusts, investment trusts, OEICS, UK REITS or a few other investments such as ETFs, then additional annual reporting on Form 8621 is required (there is a significant penalty for failing to file this) because these kinds of collective investments are treated in the United States as Passive Foreign Investment Companies; which give the potential for significantly greater tax bills in the United States (Dennis may need to check this).
From an investment perspective, we only qualify to invest in ISAs if (and only if) we are Ordinarily Resident in the UK. So someone here for only a short (under 3 year) assignment would not be able to open an ISA.
It is also questionable as to what tax advantages might be gained from investing in an ISA, because most US citizens or green card holders resident in the UK are not domiciled within the UK. So in a nutshell:
a) invest in an ISA if it promises better returns (eg interest rates are good) as compared to non-ISA savings; but
b) if you are not UK domiciled set up separate capital and income accounts offshore and you will (legally) owe zero UK tax on the interest you receive and just have to pay Uncle Sam (sadly at tax rates of up to 35% although your precise US tax rate will depend on your total income)
To turn back to meggles first question, there are indeed brokerage houses in the UK who will manage money if requested ...