Should I tile from the outside in towards the internal corner,
It's a matter of opinion but, yes (in mine) it will look much better this way and putting the cut tiles here will 'hide' any out of (vertical) squareness of the walls. If you start with a full tile in the corner (bottom row) you may need to cut a sliver off the tiles as you go upwards, or worse, have to put a sliver in, Putting a sliver in WILL look so naff and spoil any effect you were hoping for, and make even the most painstaking project look like it was done by a complete novice! Also I presume you will be using tile edging strips at the ends, if so these look so much better when against the manufactured tile edge rather than a cut one.
or from the internal corner to the outside, either way I have to cut about a
3rd of the end tiles, I can't decide which way would look best!!
Tiling is all about planning. Don't forget, wherever you put the FIRST tile determines where the rest go! An hour or so at the beginning before any tile is fixed can be worth many hours of you saying "If only I had.....". Tiling is not as easy to change as wallpaper or emulsion if you don't like it!
Hope that hrelps.
John