As you have to reclear security in the UK, even if connecting, you will have to pass the BA and/or BAA baggage police. You run a considerable risk of having it checked with the sizer.

Try it if you like, never look 'em in the eye, pull your bag on the opposite side of you from which they are standing, don't hesitate or all is lost. And don't be upset if they nail you. Happens all the time.