What's commonly done here in North America is as follows:
1. staple a polyethylene (which I think you call "polythene") plastic vapour barrier to the wall studs and caulk the bottom edge of it to the 1 inch lip of the bathtub that sticks up. That way, any water leakage can't get into the wall.
2. fasten the tile backer to the wall studs over the plastic vapour barrier. The bottom edge of the tile backer should stop just a little (1/8 to 1/4 inch above that 1 inch lip around the bathtub. The tile backer is much thicker than the 1 inch tall tub lip, so the front of the tile backer overhangs the tub lip.
3. tile the walls. The bottom row of tiles will hang down unsupported in front of that 1 inch lip around the bathtub and extend down to the top surface of the tub.
4. grout the tiles and then caulk the joint between the tiles and the tub
I'm wondering about filling the tub with water prior to caulking.
My understanding of the purpose of this is to caulk the joint when it's spread as far apart as it's likely to ever be so that the silicon is not in tension when the tub is full of water.
But, since silicone caulk is very elastic and sticks tenaciously to both glazed wall tiles and the enameled surface of a bath tub, any tension in that silicone caulk is of no real concern.
I'm wondering if it wouldn't be safer to caulk the tub when it's still dry and empty to avoid adhesion problems. If you accidentally splashed water onto that joint during the caulking job, then he silicon wouldn't stick to the wet area, and you'd be rushing to dry that spot well before continuing the job. And, that situation isn't conducive to an attractive caulking job.
If it were me, I'd caulk the tub with silicone, wait a day, then fill it with water to have a well deserved long leisurely bath, but I'm open minded to having someone changing my opinion on the matter with good reasons for caulking it when it's full of water or good reasons not to caulk it when it's empty.