The retinal scan program several posters have queried is known as IRIS. You may register for it regardless of passport- if non EU passport holder you must have passport stamps proving you have entered the UK several times in the past 2 years . There are IRIS offices in all LHR and LGW terminals airside (past security)- the registration takes about 5 minutes and you may use it within 3-4 days. No need to fill out landing cards or any other documentation. It is worth its weight in gold.