My understanding is that it is something that heads have the discretion to promise, but no ability to deliver, if the school is a LA school with the LA as the admission authority.
If the head says 'oh yes, we'll keep that place' [I have heard heads say e.g. 'Oh yes, we favour children who have been in the pre-school' when there is NOTHING in the admission criteria to support that] then if someone applies to the school for the vacant place, and the school is e.g. under 30 in an infant class, or under PAN elsewhere, the school has to admit the other person. You would then have to appeal on return, on the basis of what the head said, and you might or might not win.
Yiou might be lucky, and no-one else might apply. You might already be in an infant class of over 30, in which the school might be able to refuse admission of another child on the Infant Size Regs but you might [and that is a very big MIGHT] be able to appeal on the grounds that the class when you returned would be no bigger than when you left, and the school might offer no resistance.
Or someone else might apply, and it's a KS2 place, and the school might be prepared to take the other child to make up 30, and you at 31 on appeal on your return.
But my understanding is that they can't GUARANTEE you your place back - even if the class is well under 30, if 2 sets of twins applied, the places are their, not yours.