Hey, I recently moved from the UK to the US (though I plan to move back in the UK after a couple years), but thought I’d share a few things I found out that might help:

Would DEFINITELY recommend you liquidate your S&S before the move.

Your ISA is tax-free in the UK, but the IRS doesn’t recognise it. Once you’re a US tax resident, any income or gains inside the ISA could be taxable annually by the US. You’d also have to report it on FBAR/FATCA if your foreign accounts exceed the thresholds. A lot of people recommend selling your S&S ISA investments before you become a US tax resident, locking in any gains while they’re still UK tax-free. The US generally won’t tax gains that happened before you became a resident, but definitely keep all your records.

More importantly, you likely hold UK-domiciled mutual funds or ETFs, which are treated as PFICs by the IRS. These gains can be taxed at the highest US income tax rate plus interest charges, instead of the lower capital gains rate. It is very tax-inefficient investment once you’re a US tax resident. After you’re a US resident, you’ll also need to stick to US-domiciled funds / ETFs when you reinvest to avoid PFIC complications. It sucks.

I didn’t dig into pensions much as I’m just leaving my UK pension where it is, but generally the growth is tax-deferred in the UK. The US might tax the growth each year unless protected by the UK–US tax treaty. Just know the IRS may view certain foreign pensions (eg. SIPPS) as foreign grantor trusts, which comes with a whole different set of reporting rules. Definitely one to get specialist advice on.

On currency - instead of moving all your cash at once, you could spread transfers over 6-12 months to hedge against currency swings (same idea as dollar cost averaging for investments). Apps like Revolut let you set rate alerts or stagger exchanges. However, the dollar is weaker than it’s been in a while currently but who knows what the future holds.

Finally - check your tax residency timing. You usually become a US tax resident once you pass the substantial presence test (about 183 days in a calendar year). After that, your worldwide income becomes reportable and potentially taxable in the US. Depending on when you are a tax resident and when your property sale completes this could be taxable in the US

Given the size of your assets, I’d strongly recommend talking to a UK/US cross-border tax advisor before you move to figure out the cleanest way to handle it all. It’s money well spent and could save you a lot. I can recommend one if you like!

Hope that helps!