"U" is a preposition in Czech, meaning "At" - the local pub naming is slightly different from e.g. English, where the pub would be named "The Bear Cubs" or something, whereas in Czech, it's literally "At the Bear Cubs". And they do run their own brewery there, too, so you were correct about that as well.

As for Tesco - the banner out front is misleading, Tesco only has the supermarket in the basement.

"a sense that it may once have been Prague’s main high-street" - Wow, you do have an eye for lost history, this is actually spot-on, a hundred years ago the street used to be the high-street, then named Ferdinandstrasse (built when medieval city walls were demolished): http://www.ak-ansichtskarten.de/ak/93-Alte-Ansichtskarte/410...