Also, did Holland & Holland make guns themselves before the war?
Holland's didn't have a factory until 1893. Even after that some of the models they sold continued to be built by others. Their boxlocks, especially their double rifles, were built by Webley & Scott until very late in the game.
Just looking at your revised list in your original post, I guess it depends. Are you looking for those that made all of what they sold, or just some?
Almost all of the London makers sold guns under their own name that were built by others. Churchill sourced a lot of guns from Birmingham. Lang also had a long tradition of buying guns from the trade. Lang was forced into receivership and taken over by T. W. and H. J. Webley in 1898, due to bills from Webley for guns that they had sold but could not pay. Lang was owned by the Webleys until the company was sold in 1925 and, during that time, a lot of Lang's output were Webley products, as might be expected. This continued even after the sale. Percival's pair of Lang .450 No. 2s (sold at Bonham's last year) was bought in from, and finished by, Webley in 1927. I have a Lang boxlock double rifle in my database, Webley serial number 13735, that Lang sold in 1952. Likewise Woodward sold a few Webley boxlocks.