2 pack polyurethane for exteriors is a good bet if you dont want to go tearing up the tiles and replacing them with non -slip.

Its not too expensive , is available everywhere , very hard wearing and gives good coverage.
Just mix in some pre-washed fine sand and away u go. The trick is adding enuff sand to do the job but not too much to appear unsightly.

This stuff is thick when mixed and holds the sand in suspension quite well , apply with a brush.