Just for a bit of background as to the location of Billiter Square, which seems to have been residences in the 1700s that were converted into offices by the mid-late 1890s, rather than a manufacturing type location... it suffered bomb damage in WW2 and the area was cleared and rebuilt post-war so no trace of its original appearance survives except in archive photos
http://collage.cityoflondon.gov.uk/view-item?i=32658 - though there is a map of the area dated c. 1887
http://collage.cityoflondon.gov.uk/view-item?i=31599So, I went looking for Gustav Doring instead, and found one who in the 1891 Census was staying in a Liverpool hotel where he was recorded as a Merchant, and another who states he was a Glass Agent in the 1901 Census in London. They could well be the same man, despite the stated 3 year difference in ages... the 1901 Census gives him with Paul Doring (who may be related but is not listed as a brother on the Oldenburg emigrant database) and family, and all of them are visitors in the house of another German woman, Annie Syvarth.
Gustav Doring married Bertha Duncan in Birkdale, Lancashire later in 1891, and one of their sons was Eric Duncan Doring, who Fred referred to in the previous reply. According to the Old Bailey records Gustav Doring (occupation: agent) was summonsed in 1900 for "concealing funds during bankruptcy" but was still working as a Glass Agent on his own account in 1901 (see Census entry). His name is shown on the electoral roll for 1912 and 1913 at 66 Denton Road, Twickenham even though he is absent from there in the 1911 Census. He is shown as having died before November 1920 in Zaandam, NH, Netherlands. His widow and children remained in Hammersmith, London until at least 1931 which is when Eric was running the business at Charterhouse Chambers. Bertha died in 1940 and her probate entry states "DORING Bertha Gertrude of The Sally Lun Rottingdean Sussex widow died 21 January 1940 Probate 1 June to Eric Duncan Doring glass bottle merchant Gertrude Ellen Marguerite Doring spinster and Charnley Duncan Doring glass bottle merchant. Effects £1462 10s 3d." Eric, Gertrude and Charnley were her three children.
That they are all referred to as merchants or agents leads me to suppose they did not have their own manufacturing capability but simply bought stock in from various glassworks, to sell on. Thus the registered designs in their names would be either designs they had commissioned or may be a way to protect an imported range from being copied and sold by other glass merchants.