you are right in the sence that is you are sick while you are on leave from work you dont have to take the holiday.. (people do it where I work all the time)
I doubt there is anythign stopping you taking the holiday time as sick and still going on holiday...
that is to say if I book 1 week off work, and book 1 week in spain. I can call in sick on the monday of my 'leave' from spain and say I am sick and I am taking my 'leave' as sick..
(I think, judgin from what peopel here do, however the boss aint gonna be happy)