The newspapers from that time have adverts for that address. For example, the Sevenoaks Chronicle and Kentish Advertiser has an advert from Friday 12 August 1921.
REDLINE MOTOR SPIRIT
For cars and cycles
Efficiency and economy
Union petroleum products
Head Office: Waterloo House, 16 Charles Street, Haymarket, SW1
It's quite a prominent advert, with a little picture of an oil can, so the company must have had a fair bit of money to be able to put it in.
I guess if Waterloo House was an office, there could have been other businesses located out of there.