Indeed, that's the chicken-and-egg nature of this question since LHR/T5 etc were all designed with the no clean arrivals paradigm being the law. I presume any change in the law would be closely tied to the construction of a new terminal somewhere which could use it, probably - but not necessarily - at LHR. I'm far less familiar with T5 than most here, so will bow to more experienced visitors in the feasibility of a secure arrivals channel, but even in the event of a change in the law I suspect most passengers would be stuck with transfer security for many years to follow.