You can get tile cleaners from places like Wickes and most tile shops. It is a slightly acidic liquid, that you pour on and then scrub the tiles. It should clean them up after 1 or 2 treatments.
http://www.wickes.co.uk/Floor-Tile-Clean-and-Shine/invt/166075Once it is clean then use a tile sealer (you get matt and gloss sealers), again it is a liquid that you pour on and spread over the tiles, allow to soak for a bit then clean off the excess.
http://www.wickes.co.uk/Satin-Tile-Sealer/invt/166071Read the label on all these things, some are used differently. You also get different types to suit different types of tiles, so make sure you get the correct one!