if removing your potware (advisable imo) if they're not already fitted with isolator/service valves, fit these as they will help accomodate the raised floor height issues.
Thats great advice! Must try to remember that one
For where to start, I always think it looks nice all centred up. Centre the width of the room and dry lay some tiles to see if they work nicely to the edge, you want really half a tile cut around the room or more. Any less and I dont think it looks right, but sometimes cant be avoided. If it doesnt work out right (as in tile cuts are too small at the edge of the room) then use the other centre of the room. Im not mad
if you imagine, you have marked a centre line on the floor and put a full tile up against it, and then laid more tiles etc, thats one centre point. The other is if you mark the centre of the tile with a pencil, and then match that with the centre line mark, and then mark the floor where the tile edge is. Thats centr line number two.
Then centre the length and dry lay again to see if that way works out ok.
Then you should know where to start. Plan your route out of the bathroom, ping a chalkline (a straight line you work off) and off you go.