The imposition of VAT is not going to make a DC grammar material if they inherently are not. The reality is that this topic is one that really only affects a small very percentage of the population. They are aggrieved and trying to get buy-in from the masses with conjecture about it having much more significant overall impact than it will. @StarlingsForever
Yes, it is hilarious including the assertion that their children will take all the grammar school places because they are so much more clever and the assumption that they will be quickly be able to buy a house near a good state school and switch their child over as if the oversubscribed state school won't have a huge waiting list. @wombat15
Just because it's a minority doesn't mean it doesn't matter.
Nobody said "children will take all the grammar school places". What everybody has said is that competition for those places will become more intense, which is uncontroversial and is already happening. Just ask a tutor. The result - unless grammar schools expand, some kid or other gets nudged out and doesn't make it. You're right we can't assume private school children are "so much more clever" and will take all the places automatically, but there's a longstanding complaint about grammar schools that the 11+ is dominated by expensive tutoring. If that's even partly true you should assume grammar schools become even more difficult for the less wealthy.
Nobody said, either, that it's easy to get into an oversubscribed state school. What we've repeatedly said is families that can't easily move and don't have an reasonable state school vacancy might stick it out and pay VAT to the next point-of-entry, and THEN will (following the same theme) join an intensified competition for the oversubscribed school, on an equal basis as everyone else...and again, some kid or other gets nudged out. In this case you should simply assume the wealthier families "win".
Either way you're left with the economic fundamentals. The country has fewer children getting an education of their parents' choice and more children (typically, the less-wealthy) being forced into less-preferred settings. You also have less expenditure in a sector of the economy that's highly tax-generative, combined with a reduced incentive for higher earners to earn. And you have a bunch of redundant teachers (who can't be assumed to match location, motivation and skillset for state school hiring).
So those are the nuances. I appreciate your urge to crow because you think you're "getting one over" the beastly rich, but actually it would be far-sighted and public spirited to join the opposition to this dimwitted tax.