When I moved back to the US from the UK, I just left my UK current account in place. Ten years later, I was very glad to have done this as I found myself back in Europe, with some payments coming in in sterling. I did notify my UK bank of all my address changes, and have continued to receive statements - though now with Internet access to the account, it's not absolutely necessary. I wouldn't bother with the "international account", especially if there are fees for it.

Don't know about the ISA account, though I have a US based IRA, which seems to be similar. Technically, yes, you'll have to declare the interest and pay US income tax on it. You can always decide later to close it out and transfer it elsewhere.
Cheers,
Bev