At BNA newspaper references to "Claude Harris Ltd. 122 Regent Street" begin in June 1912.
Throughout preceding years there are items from Negretti and Zambra at this address, advertising barometers, thermometers, binoculars instruments. In later adverts they are 'Scientific Intrument Makers. This matches with the valuation records dated June 1914 where Negretti and Zambra are recorded as 10 y leaseholders from October 1909.
The property description indicates that it extended over four floors. The third floor includes
Darkroom fitted 2 sinks.
and the top floor:
Studio (top and side lighted). Small Dressing Room partitioned therefrom & 2 other Apartments.
Negretti and Zambra were still advertising from that address after the Claude Harris records begin, and up to the end of 1913 (I haven't looked any further).
So it seems that Claude Harris was probably occupying the upper floors of Negretti & Zambra's premises.