So as a UK resident investing in individual US shares as a named individual through say interactive brokers and having filled out the W8BEN optimiseaccountants.co.uk - W form instruction IRS foreigners US tax withholding
I'm liable for this 40% estate tax or not?

Here is what I learned from my research on this. Bear in mind I may have been misinformed or out of date on a point or two.

You are liable.
The W8BEN doesn't make any difference. That's mainly to let the broker know what dividend withholding tax applies based on the tax treaty (if any) between the US and your country of domicile.
The tax treaty in question may also have something to say about double taxation on estates (testamentary trusts), but the general rule for double taxation treaties is that they merely prevent you from having to pay the tax twice.
Your total due (the sum of the amounts going to the two jurisdictions) is generally the higher of the two amounts requested by the two countries. (with exceptions)
i.e., you generally get a full tax credit in one place equal to the tax you already paid to the other place.

The general rule is that US inheritance tax is definitely due for any deceased natural person globally who owned US assets at the time of death.
(there is a fancy definition of what constitutes a US asset, but it generally includes the US-listed stocks of US-domiciled firms, including Berkshire, and US real estate)

You may further wish to note that some brokers, *if informed*, will require the estate of a non-US person to complete US probate before releasing funds.
e.g., Schwab, which is why I closed my account there.
It is illegal for the broker to require this (which they more or less agreed with), but they do it anyway.

But...
It should probably be noted that, though this tax is generally due upon the death of a non-US-Person holding US shares, almost nobody pays it.

The largest entirely legal loophole is to hold the shares through a holding company. A company doesn't die, so the decision has been that no tax is due.
I had a company in the British Virgin Islands for a long time, for this very reason.
The usual route is to have the holding company in an untaxed jurisdiction, an international business company (IBC) in the jargon.
This does not get you magically out of any other taxes, as some shrill reporters dumping on "tax havens" might suggest.
Rather, the income taxes paid on profits is then a function of the domicile of the shareholder, not the domicile of the company itself...they aren't avoided, although depending where you live they may perhaps be delayed.
The main disadvantages are the cost of running the company (maybe $750-$2000/year), having to pay more for your data subscriptions because it's a corporate brokerage account,
the incremental paperwork hassle because of the first two things, and the much bigger hassle that some IBC jurisdictions now require a "real local activity" to let you have a tax free company.
There are some excellent super simple loopholes for the latter though, if anyone is interested.
Also, dividend tax may be higher.

However, perhaps more to the point, almost nobody outside the US actually pays the US estate tax even when it is due.
Look up how much is collected each year under this tax...the total is about the size of one really rich person's portfolio.
In essence, the US does not know when a non-US person dies. Therefore the US doesn't ask for the tax, and the individual doesn't take the initiative to offer it.
A typical situation: a married non-US couple has a joint brokerage account. One dies. The other simply withdraws the money without telling the broker that one holder died. Done.
Many non-US brokers don't care even if they did know.
This approach does not meet US law, but it's what people do.

I wouldn't feel good recommending this as your chosen course of action, but it wouldn't feel all that bad either.
If you are not American, the moral qualms of not paying the tax that the US deems due may be modest. Based on the stats, it seems most people feel that way.
In general, not paying a tax the US deems due may be against US law, but is probably not against the law where you live.
A lot of us do things every day that are, under the laws somewhere, unlawful. The invasion of Ukraine is a war, and the king of Thailand is self-enriching absentee.

Jim