Thanks! Unfortunately I think this prevents me from opening a US domicile account through HL, of the sort suggested above: http://www.hl.co.uk/funds/terms
You will find it almost impossible to open a US domiciled investment account without a US address. However, you can buy US domiciled Vanguard ETFs through UK brokers and platforms like H&L....the account is in the UK, but you buy an ETF that is traded on US exchanges that is therefore not PFIC and it is also HMRC reporting because the right paper work has been submitted by Vanguard. If you or your wife already has an investment account in the US you could buy the same funds through that as well and better still you could do it inside a ROTH IRA if your wife has earned income on her 1040 and avoid US and UK tax forever, but you should be aware of the withdrawal restrictions if this is to fund college....the restrictions aren't too bad though.
But does this mean that I can open an account with a US broker, or even a roth IRA? I've noticed that their websites tend to ask whether I am a 'resident alien', and I'm not sure what to say: as far as immigration, no; as far as tax... yes?
As I said opening a US account without a US address is almost impossible.....almost is important there...I have heard that Charles Schwab UK will deal with US expats and open ROTH accounts in the US.
Are you sure about that? My understanding is that the UK-US tax treaty specifically allows for retirement schemes.
The tax treaty allows for some good and sensible tax treatment of crossborder pensions. However, that does not change domestic tax laws or filing requirements. Guya did not say that you would necessarily pay more tax, but you filing would be more complicated. The exact US status of a non employer UK pension account is debatable, but the safest thing to do would be to treat it as a foreign grantor trust.
The main point is that she doesn't really earn anything in the US (just a few thousand a year), and I don't earn anything in the US. Therefore (I think) precisely because UK tax rates are higher, the tax I've paid on my salary is credited towards what would have been her tax bill. (She also gets to claim me as an exemption.) She files on the arising basis in the UK.
I think your best options to fund your son's college would be
1) ROTH IRA with existing US brokerage if you have one or Schwab UK
2) 529 plan set up by US grandparents that you gift them money to contribute.
3) US Vanguard ETFs bought through UK broker
4) UK savings bond/junior ISA...low risk and low returns and US taxable.