It is possible to register for the retinal scan at LHR T1, just past security in International Departures, and use it coming back into UK in lieu of queuing for passport. It think it is for UK (EU?) passport holders only so if yoou need to pass immigration it may not be for you.. Yes it does work but with only one channel that is not always serviceable the benefits can be minimal. The channel is to the right at the endo of the "Immigration" desks and it is not until you go there you are aware of whether or not it is working. By that time you may have to return to the "normal" queue which may have (inevitably) grown in your absence!