It’s the Motor Scout, developed by Frederick Richard Simms in 1899, most likely the world’s first armed vehicle powered by an internal combustion engine.
(I recognised it immediately, but had to look up the name, which entailed removing about 10 cm worth of books and an almost as thick layer of dust to get at my copy of The Illustrated Encyclopedia of Military Vehicles by Ian V. Hogg and John Weeks, 1980, which I was 100% sure had that same photo of it — on pages 6–7, to be precise, where it’s captioned as the “Quadricycle Maxim Gun Carrier” being demonstrated at Roehampton in 1898.)