Witham, 200 Greenwich High Rd, Greenwich, London

Do you have a source for this? Looking at Google Maps there is currently no such address, but if it ever existed it would have been somewhere near what is now the site of the Ibis hotel on the corner of the High Road and Stockwell Street.

Further towards Deptford, near the junction with Greenwich Church Street, there used to be a place where I had a magneto overhauled in the early '80s (but I don't recall the name).

There used to be a Cooper in Greenwich, as i took him my BTH magneto to repair back in the late 1960's There was also another dealer that sold vintage bikes at may be the high street, and he would be listed in the older Motorcycle and Motorcycling magazines. He was an American living in Greenwich

Early 70's I worked for Stewart and Ardern, who were Main Distributors for the BMC car group, and was based in the Central Parts Warehouse in W12. One of my sales colleagues had a regular customer named Withams who I think was a small BMC dealership. I can't recall their location, but we never went anywhere near Greenwich. 
No real help I know, but possibly a related company and may open up a thread to follow. Or just another Internet rabbit hole... 
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The address looked like a bomb-damaged site in 1952. No.202 is missing and 200 has lost its upper stories. The occupier looks to have been Charles Hibbs ? Boarded-up in the photo ? Might there have been a more modern structure that replaced it prior to more recent road widening ?

https://m.londonpicturearchive.org.uk/view-item?i=66169&WINID=1740082887910

I know that Witham appears in the Norton factory records. I'd assume that the club officers could tell quite easily from the database just how often and between which dates which may give some clues.

A local library or museum might have trade directories and telephone books.

I have noticed in the pre-war ledgers that Nortons sometimes sold machines to car dealers. It's also possible that Witham was an export agent. Where is the bike now ? Where did the first owner live ?