Best grout for tiled shower over bath

Hi,

I have just finished ceramic tiling bathroom, walls & built in bath with power shower over. The tiles were adhered straight on to Wickes Tile Panels which are fixed to timber battens which in turn are fixed to the brick walls.

What will the best grout be to use? I have looked at Epoxy grouts as I wouldn't need to seal afterwards but would a Mapei Ultracolour or similar be ok as it claims to be water repellant and used in swimming pools? Cost is not a problem but just want the best grout possible for a keen DIYer to use which will last.

Also, can I grout where the tiles meet the sloped ceiling or will this need to silicone'd? The tiles are 7mm x 150 x 150mm plain white ceramic and there is a 3-4mm gap where the tiled vertical walls meet the ceiling.

 

Thanks, will there be any water ingress with Ultraclour? I'm determined to make sure no moisture gets behind the tiles. I stupidly negelected to prime the Wickes Tile Panels before tiling and I used UltraTile Pro RS adhesive which is cementitious. I'm worried that if moisture does penetrate that cement could react with the gypsum boards.