Of course she can control her bowel, she can't go well!
Welp, honestly...would you always want to be constipated? How uncomfortable for the poor thing. There is a stool softener for cats...and changing her food (although I know you mentioned her food issues) is probably necessary.
She most likely needs more fiber to help create bulk to help move the stool through, unless she actually has a real disease related to the constipation...one of mine does, so I know more about cats and constipation than I even want to know.
If your cat doesn't always poop in the box, it's because she relates the box with the pain of pooping and goes elsewhere.
Not only do they have a stool softener that can be given, there is also a prescription med. (here in the US it is called Cisapride).
And plus, if she likes hairball medication (most pet stores have it here in the US), just put a little on your finger (read the directions on the tube) and she should lick it off since it comes in fun flavors for cats like tuna, and plain malt flavor. If not, you can wipe the dab on her paw and she will be forced to lick it off. That can also help some in mild constipation cases.
That's a cheap place to start...plus I would tell your vet she is always constipated right away.