It can be done both ways,but here's something to keep in mind. If the floor tile is laid first and the wall tile comes down over it the floor tile acts like a flashing with the wall tile on top to redirect water away from the joint.
If you lay the walls first then cut the floor to the walls even though it is siliconed the integrity of the joint is compromised and over time water will enter the horizontal joint and move under the floor tiles.
Obviously it isn't critical in open floor areas that don't see a lot of moisture, but in the confines of a shower recess it is critical for the water to be deflected away from the joint.
Here's another example, You wouldn't dream of tiling the walls first then installing a shower tray pushed up to the tiles? then say the Silicon will hide any joint and it should be fine. Water will indeed find its way between the tray and the tile. Your tiling is no different.
Down here the Australian
standard states that wall tiles must come over the top of the floor tiles for the very reasons that I state above.
If you are in doubt as to what you think is the correct method you should consult a copy of your local british standards.