Hi,
I cant measure mine at the moment as it's at the bodyshop getting painted but I always felt that mine was on the low side too so I looked in to it further, now, I cant quite remember the figures that I found but it goes something like this...
The centre of the towball must be between 230mm and 350mm from the ground which is quite a range but depends on vehicle.
What I can tell you is that mine is very close to the bottom limit even with new ZF springs on.
I was worried about boarding the ferry to France as some of those ramps are bloody steep but it was fine so you should be ok.
Just be mindful of the way you load your car and caravan/trailer I go for a nose weight of around 50/60kgs as I find it makes for a better more stable, comfortable tow. Try not to fill the boot with heavy items either, stick them over the axel in the caravan/trailer.
However, if you're traveling 5up,boot full, caravan/trailer loaded to the max then you might benefit from some kind of modified suspension setup but wouldn't advise that, rather just take less stuff/people

Simon.