How do these work. I am particularly thinking of the case where people
have
stopped in them to turn right?


You should NOT enter a box junction unless your exit is CLEAR. And you
should be prepared to SLOW DOWN and STOP before a box junction if your exit
is NOT clear.

If you found yourself stuck in a box junction, could you just put on your
right indicator to try and "save" yourself?

If you follow the rules, you should not get stuck in a box junction. If you
do, then you've failed and expect to get the necessary punishment for your
failure to know the highway code.