On the 777 into LHR you are 180 kts until they slow you to 160 to 4 DME. Going downhill ( on the glide slope) around 500,000 pounds landing weight it won’t slow down to or hold 180 at flaps 15 gear up. Your choices are flaps 20 gear up or flaps 15 gear down. Most of us were trained to put the gear down before you go to flaps 20, mainly because if you go flaps 25 before the gear is locked down you will get the config warning.