+1 to OAP.
To the OP are you resident in the UK for tax purposes. You can be domiciled in the UK, but not resident, just make sure you understand your status fully.
Remember that although the US will tax any gains you have on a UK cash ISA, UK tax credits may well wipe out any US tax due.
I've looked into how I'd invest my money as a US citizen in the UK and the simple option would be a long term savings account inside an ISA. As there's only a savings account you avoid the issues with foreign pooled investments and you can still get "good" interest rate or around 4%.
If you have good book keeping skills there's nothing to keep you from buying individual stocks and bonds, there's no tax penalty for that, but the reporting would get complicated.
Finally the situation with UK pensions is quite complex too. It's good to have one, but you should definitely get your taxes done professionally as a Tax Treaty position has to be taken to exclude contributions from US tax or you have to pay US tax on the contributions up front.