And what exactly was the point of depositing into the ISA?
To save tax money.
And what OTHER ways could you achieve that same aim? Perhaps some of the other methods will not have the serious drawbacks that this method has: namely,
(a) forking your dough over to some faceless entity that cares more about their sales then about the growth of your money (i.e., that's why they're charging you commissions for the privilege of hoarding your money rather than an annual asset fee)
(b) exposing you to riotously large US taxes on unit trusts and investment trusts (this assumes you're not doing a cash ISA)?
You're posting the question here, so the chances are you are not Domiciled in the UK. Domicile is where your father is from and/or where you are from; it is not where you live. Where you live is called Residence, and for UK tax purposes, it's completely different from Domicile.
Well, if you are not Domiciled in the UK, there are a number of other ways to achieve that goal. and many of them are much better.
So, the next question is what do you plan to do with the money you set aside? If you plan to return to the USA, and not likely to spend the money in the UK while you are living in the UK, then the best place to put that money is: ANYWHERE EXCEPT THE UK. This is because nondomiciled people don't pay tax in the UK on their interest and dividend earnings outside the UK, for so long as that money remains outside of the UK. And as long as it's anywhere but the UK, you can top up your US accounts with this money, or start a new one in the US.
The usual advice I recommend is that you first make sure you've maxed out your contributions to your pension plans. Next, invest in the USA, in normal (not IRA/ISA) accounts.
Other factors that affect where you invest: Saving for a house in the UK, saving for school, saving for retirement, whether your pension plan is in the US or the UK, whether you are married or not, etc.
If you email me directly, I could give you a personalized investment plan that covers the lowest tax savings strategy for you (but I do charge for my advice). If you write here, I will give more general advice that you would still find very useful, but remember: tax laws change. Advice given this year may be completely different from the advice I'd give someone three years later.