When you show me the price in GBP I assume the tax has already been added. That's only a thing for the US.

In the US, sales tax is handled on the state level not the national level. So it varies wildly.

To expand on that: it's on state, county, and city level - I believe all those jurisdictions can introduce a sales tax.

the store is able to compute it on the fly at the register, so they can also print price stickers* (for products that don't have the price on them)

showing prices without vat should be banned for sales that are never business-to-business.

* unless they also vary wildly by the week

I was talking more on the lines of exemption brackets. That's where the concept of xx999.99 price tags came from, rather than anything psychological.

Or if it's business-to-business, at least if the UK is anything like here in Denmark in that regard.