BR were trying, by various degrees, to make some branchlines - East Anglian ones in particular - viable, or rather less lossmaking than had been the case with steam, so anything that was cheap to procure and capable of carrying low-volume passenger traffic (and it was very low volume for some routes, with some services regularly failing to reach double figures in terms of passengers carried) at relatively low cost had to be a goer.
That was good news for AC, Park Royal and Waggon und Maschinenbau in particular, all of whom turned out 4-wheel railbuses.