No, all stacks have a protected zone between certain level bands and those aircraft that have to hold further out also hold at higher levels so no infringement occurs. Also, if a level is being used that is unusual for the stack then we have written down which of the other stacks are infringed and therefore the particular level cannot be used there.
For instance, the upper level at Biggin for general use is FL150 as FL160 and above cannot assure separation from Lambourne, so Lambourne uses FL160/FL170 and Biggin doesn't. Or something like that anyway, I'm sure HD or another LHR controller can give the specifics better!