This is the problem with people that stick a load of defunct pubs on and say NOTHING in the comments to give it context. Mr Mason is very good at this.
The Britannia was a pub, in the 1870s, on the High St of Maidenhead (and i assume on that spot, it looks like it is from the historic mapping) - ironically, it is now a Britannia BS, and that is where the confusion lies.
It is irritating and lazy that people can't be bothered to say a basic comment to let everyone know.
