For LHR you do NOT even need a square available, they will generally stamp anywhere and favour making multiple UK stamps on one page (I've got NINE on one page, for example), just go to the UK and swing by the embassy on Grosvenor Square when you have half an hour, it will be free and quick. Please notice that every single US FT'er that travels frequently abroad gives this same advice. Don;'t mess around with it at home, just not worth the hassle.