Older readers will recall the Selous Scouts, the special forces of the Rhodesian Army. Nowadays there is a Selous Game Reserve in Tanzania. Both are named after Frederick Selous (1851-1917), a British explorer and big-game hunter.
The Oxford Names Companion suggests that this surname is English and ‘of uncertain origin, perhaps a habitation name from an unidentified place named with the OE elements s(e)alh willow + hūs house’. I would have thought that a name with that etymology would be more likely to be pronounced ˈseləs (cf. Backus from bǣc ‘bake’ + hūs , Malthus from mealt ‘malt’ + hūs ) — which it isn’t.
But maybe that etymology is not correct. Wikipedia states here that Selous is the Anglicized form of the Dutch name Slous. If that were the case, I would expect it to have been pronounced in Dutch as slɔus, which might give English slaʊs. Wrong again.
Very much more plausibly, Wikipedia tells us in its article on the big game hunter that the name was originally French.
Frederick Courteney Selous was born on 31 December 1851 at Regents Park, London, as one of the five children of an aristocratic family, third generation of French-Huguenot heritage. His father, Frederick Lokes Slous (original spelling) (1802–1892), was notably Chairman of the London Stock Exchange.
As a French name, Slous would be expected to be pronounced slu. Equally, the spelling Selous implies səlu. And French schwa is a very variable entity: cela can be sla or səla. This origin would satisfactorily account for the English pronunciation səˈluː.