You can have an ISA if you want:
1. It would go on the TD F 90-22.1 as a foreign securities or bank account.
2. If it holds stocks & shares then the dividends go on your US return on Schedule B (and on a Passive Form 1116).
3. If it holds cash then the interest also goes on your US return on Schedule B (and on a Passive Form 1116).
4. If it holds unit trusts, investment trusts, UK REITs, ETFs or other collective investments then these get reported as Passive Foreign Investment Companies on Form 8621 as well on the return. Generally speaking, holding these kind of investments make US tax really complicated!
5. Many readers of this forum could invest offshore or in the US without the limitations of an ISA because many are not domiciled within the UK so would not pay UK tax on interest, dividends & capital gains if the investment income was earned outside of the UK and not brought to the UK.