and having just started you have gone in at the deep end,especially if they were brickbond and even worse brickbond B+Q.
check your walls first for flatness forget level it means nothing,flat is what is important,you only need to worry about level on the external cornered walls.
many bathrooms wont have a external cornered wall apart from the soil stack and the window.
10mm trowel will be needed but as stated before check for bowed tiles,a way to make sure a 1mm bowed tile is stuck is to back butter length ways making sure your trowel over laps the 2 edges of the tile personally i do this anyway with anything over 300x300,pull off a tile after you put it up and it may need a bit extra across the middle,but using a 10mm trowel and back butter you should hit the adhesive fine.
if the walls are not flat you have no chance with brickbond you will always get lipping,so its a case of do i do the job or not.
soppy bloody plasterers will always tell the punter the
tiler will get over that.
always remember its his job to make the walls flat not yours if you go over 12mm of addy even with many SPF you are on dodgy ground,the first tile needs 2mm more the next tile needs 4mm more and so on just to keep the wall flat.
so always check for flatness and know the tile size when quoting.