Is there a law that requires consumer prices to be stated incl tax in the UK?

I have this fear it was an EU law is being non assimilated as we speak

Misleading pricing is one of the most annoying things about places outside Europe 

Yes, all UK consumer prices must include VAT.

Just check that website doesn't have a B2B as well as a B2C landing page; if you clicked through from a link you may be on the pro site.

Also, some cheeky .uk sites are in fact based in the EU, and you end up getting spanked for import duties if you don't read the ts&cs carefully enough.