A rational company optimizes profits, rather than just revenue, when your costs go up, you may be interested in raising your prices, even if it reduces the number of items you sell, because part of the revenue has to be paid to someone else. You can see this trivially if a business only has a 1% profit margin on an item.

It's likely going to fall on some combination of business and consumer that we won't know a priori.

Is Argos exempt from this tax? They certainly exceed the 500m in revenue threshold.

If they are, this tax seems to be more like a tariff than not.

Argos isn't an online market place.