Johnsons do a clear liquid polish, it's in the supermarket with other polishes-sorry can't remember the name, after cleaning the floor you can either mop it on or get down on your knees. You lightly spread it over the floor and then leave to dry, this'll take some time. The first time you use it put 2 thin coats on making sure 1st coat is thoroughly dry.
The finish does wear off but you just keep repeating the process every week. Not only do the tiles end up with a very high gloss but it also makes it easier to sweep the floor.