There is shortcut for switching the chapter to the next one, but there is no option to set it up for changing the titles.
I've tried to play a few blu ray disks with smplayer - it's just painful, terrible. Sometimes there are 180 - 220 titles on a disk and like 3 - 10 chapters (but they have a shortcut), and I press alt+B, then the right arrow, then 100 times the arrow down, then enter, repeat from alt+B, repeat from alt+B, repeat from alt+B, and like that 180 times.
Like it's not a completed torture yet there is a bug - after selecting about 60th+ title, the title list starts to slide down (the list popup itself, not the selector in it) by a bit, the further you go from 60th (or something) the further it slides (you should select the titles, not just scrolling them) down the screen until it becomes invisible completely. For example, I'm searching for the title, selecting 130th one - "nope, not the one", I go alt+B, right arrow, repeatedly press down to pick 131th title and from some point the title list floats back up from below the window and I finally can see what title I'm selecting (115th, for instance). If I go from the last one (scrolling with arrow up) the list can be invisible, or like 3 lines high and it stretches as I keep scrolling up. The same with the mouse scrolling, it's just even worse.
Please add the option to set a shortcut for cycling through titles.
p.s.
Version: 18.10.0 (revision 9144)
Using Qt 5.12.0 (compiled with Qt 5.11.2)
Using MPV 0.29.1
Manjaro