In the U.S., Apple advertises the before-tax prices of its products, just like everyone else.
Because the tax varies from state to state - and if I understand correctly you often don't pay tax on internet purchases.
Why not advertise the pre-VAT price of its products in England?
Because all of the UK (not just England) has the same tax, and people like to know what they'll end up paying.
Is it illegal to advertise the pre-VAT price in England?
You can only advertise pre-vat price if you're selling to businesses. If you're selling to consumers you must show the with-vat price.
After all, the tax on products varies from country to country.
Well, it doesn't vary within the countries that make up the UK.
And the rule in Europe is that you pay VAT in the country you're buying from - so it doesn't make sense to advertise pre-VAT prices for cross-border sales. Sales tax is charged on internet sales.
The actual price before taxes is what the product's advertised price should be. This way, England can do what it wants with the VAT tax. But Apple's advertised prices will be the same everywhere in the world.
Actually, most countries advertise with-tax rates. It's the US that's out of line, if anything.