Yep, every one quotes Roy Tebbutt, who says what happened, but never really why: Why would the British develop a BP round in 1887/88, after the deployment of the Lebel? They and Rubini were working hand in glove with Nobel and his new Balistite propellant, (that got Nobel kicked out of France) Ennyways, I need to go dig some of this stuff back out. Never really unpacked after my retirement . . .
Not sure who Roy Tebbut was--Navy maybe? My info came from British military documents dated 1888, 1889, 1891, 1892, 1894, 1895, 1897, 1898, 1899, and 1901. Some of the photos showing pellets of BP are dated as early as 1887. FWIW, in the primary documents I have and those I've seen, Nobel and variations on Ballistite weren't mentioned until 1890.
As to why they'd go with a BP load after the deployment of the 8mm Lebel, there were technical issues with the 1886 Lebel rifle. The 1888 and 1889 documents note problems with the rifle including poor gas handling when--not if--cartridges ruptured, slow loading, and lack of a mechanical safety. More importantly, problems noted with the cartridge and load included pressure excursions in high temperatures (tropics etc), rapid bore erosion, and dis-satisfaction with the bullet in the Balle M load. The French didn't really have a good cartridge until Balle D came along in about 1898.
The British plan seems to have been to use the BP loadings as a test-bed for the rifle, in hopes that a suitable smokeless load could be developed. When they had a workable powder, the rifle had many of the bugs worked out already.