The fellow who did the planning for Overlord was British Gen. Frederick E Morgan. He'd started the job in 1943 as Chief Of Staff to the Supreme Allied Commander (designate) i.e. some bloke who was supposed to be George Marshall, but turned out to be Ike. Given his staff of 320 officers and 600 other ranks the Long Room must have been quite crowded!
Link here ... including the three principal locations in London.
Fred Morgan The plan was in already in place and had been approved, when Ike and his own COS Bedell Smith arrived at Bushey House and COSSAC was absorbed into SHAEF. Montgomery as Allied Land Commander accepted all the details with the exception of the numbers of troops to be involved on the first landings, and the width of the coastline to be attacked. In essence he beefed up the airborne attacks and put a lot more divisions ashore on five beaches instead of Morgan's three.
I'm fairly certain Churchill's underground range was established well before the war; he conducted all his criminological and ballistic work down there, including bullet matching by photomicrograph. He did indeed train a lot of the James Bonds of the time using the instinctive pointing method devised by the Shanghai police.
As a by the by it is said that when Lord Haw Haw heard that Churchill was teaching The Home Guard how to shoot descending parachute troops up the arse with Ross rifles, he protested that this was no way to treat brave men!
Eug