As a Non-res of the UK you can hold on to any previous Cash or Share ISA's that you opened and contributed to before going down the NR route.
However, you cannot add to these or open new ISA's as a Non-Resident.
The tax rules under Uncle Sam seem worse than the UK if you are a US Citizen. Whilst I was over there I looked at ways of reducing tax and there were ways such as Municipal Bonds but you run the risk of the Bond Issuer being able to repay the Capital at the end of the term.
If you earn income outside of the USA, it is very hard not to get stung for tax by Uncle Sam and if you forget to inform them of accounts held overseas the penalties are extremely severe including a penalty of up to 50% of your capital invested in the account.
USA, a nice place to live but I prefer countries that do not tax on Worldwide Income.
Lastly, are you claiming your Personal Tax Allowances from the UK, you are entitled to these as a British Citizen.