It would seem that the booklet you have is from the 1929-1932 time frame. The Leadenhall Street address is Royal Mail House, which was White Star's principal office from 1 July 1929. Before that, the principal office was on James Street, Liverpool, and the London office was Oceanic House on Cockspur Street.
On 4 November 1932, as the Royal Mail Group was being dismantled, White Star's registered office was moved out of Royal Mail House and back to Oceanic House.
Sources: Anderson's White Star; Mallett and Bell's The Pirrie-Kylsant Motorships.
The Leadenhall St address may not have been the main office in 1912 but it certainly was in use. It's on a letter to Francis Browne dated April 3rd 1912. Browne's ticket came from the Queenstown agent.
The booklet I have is a 7 C'S Press reprint; but the front cover bears the date May, 1913.
The Heading on the back cover reads: Principal Offices of the White Star Line. Under the sub-heading: Company's Offices; while the first address is 30 James Street Liverpool; there are in addition to the 2 London addresses 22 others; for England, Europe and North America.
This list is followed by one listing: Agents; for which Paris and Cherbourg feature most prominently.
I also have a 2nd booklet (also a 7 C's Press reprint) for January, 1912; which gives First Class Passage Rates for the Titanic. It also lists both London Offices.
I didn't know about the earlier Leadenhall presence; thanks, Lester and Dave, for that info.
Odd, isn't it that the site of a one-time White Star office would later become the location of the firm's corporate headquarters?