Does the UK include VAT in all listed prices? I don't recall it working that way in continental Europe, and it certainly doesn't work that way in Canada with our equivalent (nor in the US, as you discovered).
That said, advertised prices are commonly not "round" in the first place. In Canada, we simply round the figure on the bill when paying cash.
UK and EU law requires advertised prices to include all taxes.
The most common exception is things aimed at business buyers, who won't pay tax -- so the Dell consumer website shows laptops prices including tax, and the business site shows them without, but labelled as such.
It means if I buy for business from a consumer site, and I'm logged in with my business account, I see 'ugly' prices like 5119.20, as the marketing-friendly number of 6,399.00 is chosen including tax.
Yes, prices advertised to consumers must include VAT. UK advertising laws mandate that the price must include all non-optional taxes, fees, etc. Prices advertised exclusively to businesses can exclude VAT.
That said, advertised prices are commonly not "round" in the first place. In Canada, we simply round the figure on the bill when paying cash.