Visit Britain Shop Website

I suspect it's the U.S. version of this one:
http://www.visitbritain.com/en/EN/
As everything's priced in dollars.

It is, and I found the US site through that link.

I am curious in part because as I was studying the Britrail rail passes the visitbritainshop.com was not listed as one of the "Britrail specialist agencies." Plus, those partners have the 3 day London Plus for $204, while Visit Britain it listed for $188.50.

I also found some conflicting information with the Visit Britain website saying that a London Plus rail pass could be used on the Gatwick Express train without counting against days, while other vendors say that it would count as a day, used.

From the Britrail website: "All BritRail Products are managed by the Association of Train Operating Companies on behalf of the Train Operating Companies of Great Britain."

Intriguing, I'd always been led to believe that Britrail was a Canadian owned company based in North America and not anything to do with National Rail in the UK but I see that their logo appears on the Britrail site.

To answer your question though, Visit Britain is a genuine site:

VisitBritain is a trading name of the British Tourist Authority, which is a non-departmental public body funded by the Department for Culture, Media and Sport.

So it depends if you think a government funded body is legitimate and reputable ....