Trouble is it's not a "small" tax. A flat rate of 40% goes against all economic/behavioural ideologies. When you're faced with a huge leap from 0% to 40%, you're going to take action to avoid the 40% portion. It's simply human behaviour - the "tipping point". You're going to look at your estate and do "something" to remove the portion over the threshold.
Fair better would be to have a lower rate starting at a lower amount, instead of 40% from £325k, make it 20% from £250k and then 40% from say £1million, if you only want two rates. Or even better have a starting rate of 10% and then 20, 30, 40 bandings.
For it to work, it needs more people paying it, more people being brought into IHT. As it stands, so few pay anything anyway, and they are highly incentivised to take tax planning actions to avoid it completely.
Since Rachel's budget, I've had numerous clients suddenly showing interest in IHT, mostly because of her bringing pension pots into IHT. Some of those had chargeable estates slightly over the threshold, but previous projections of IHT had shown pretty small IHT liabilities, typically £10-£25k or so, where the costs of setting up trusts or transferring assets to children etc., would have cost a few thousand, so really wasn't worth it - i.e. not breaching the "tipping point". Now some of those same people are facing IHT of £50k - £250k because of pension pots of £500k-£1m, and they're now queuing up to set up trusts and transfer assets asap to reduce their estates to get the IHT back down, but not down to £10-£25k, they're aiming to zeroise IHT! So, by trying to get more IHT, Rachel could end up getting less because it's just encouraging more people to undertake IHT planning actions!