Part of the W-8BEN declaration is that you are not a “US person” (which means subject to US tax, not related to residency) and you will have to contact your provider to cancel the W-8BEN declaration when you become one.
The whole point is that instead of the usual 30% US income withholding tax, as a UK tax-resident you are allowed a reduction of that withholding tax down to 15%. When this no longer applied, you’ll be subject to the full 30%.
I’d slightly add to what @DNM said, “terribly disadvantageous” doesn’t even begin to describe it. Some UK ETFs conform to US accounting rules, but there’s basically no realistic way of owning any non-US ETFs that don’t if you are a “US person”. It’s fine to own UK stocks, as long as you can find a broker willing to look after your UK account when you’re no longer a UK-tax resident, and any US stocks and ETFs are fine. Having fractional US stocks might well make your Freetrade account (if you’re allowed to keep it) ineligible too if it’s considered an investment wrapper.
If you only have UK stocks, you’re probably OK if Freetrade will allow you to stay. Often that that, it’s probably easier to transfer to something like HL who according to what I’ve read don’t seem to mind if you’re no longer a tax resident (even if you’re in the US) or IG who expressly allow it, but add a monthly surcharge. I think it’d be safer just to transfer all your UK assets into something like this, and then forget about it until you move back to the UK. If you sell anything whilst you are a “US person”, you’ll still be charged US capital gains tax, which is a massively complicated topic in itself.
That said, there are so many cheap trading options in the US, if you’ll be trading US assets you’ll be better off using those instead.
Disclaimer: I’m still in the UK, but read up what I could as I also might be moving to the US in the future. That said, information this way round (UK->US) seems quite hard to find, it’s mostly written for US citizens moving to the UK. TLDR: they’ll tax you any way they can, and there’s no point owning an ISA of any kind if you’re a US citizen as the US tax on all income earned worldwide.