It was BR policy, which came down from the government, to give out work to outside contractors, and several of them, at the time, for "new technology" (ie dmus, electric units, etc). This was not only because the Conservative government of the era favoured private enterprise, but was seen (and described as such) as a way of getting the commercial companies into the new technology, and into export markets. The extent to which the "export or die" attitude prevailed then probably cannot be appreciated now.
Because the main rail workshops had made BR reasonably self-sufficient in rolling stock construction (although they had always patronised the private builders to an extent), the latter did traditionally do much of their work for export, where the UK was a world leader up to 1939, but it was slipping as diesel and electrics came along. So the dmus were parcelled out among the major builders, like Met-Cam, Birmingham RCW, Gloucester RCW, etc, and also a few minors as well - Wickham of Ware built only a handful of dmus for BR, but were an established rail export organisation for them.
The same applied to electrics. The first 25Kv locos, or power sets for emus, came from English Electric, Metrovick, GEC, etc, and were built by different workshops. It was felt to be giving a helping hand to all these separate companies with developing their technology. The well-publicised transformer failures on early 25Kv emus in fact only affected one of the several manufacturer's products. If you read the formal accident report into these (one of the explosions caused a fatality) you find it giving repeated emphasis to the need to support the exporting manufacturers, almost at the expense of designs that sometimes went wrong.
It did get a bit extreme. For the railbuses, really for services which were quite unviable and unsuitable for the railway anyway, five different designs and manufacturers for just 22 vehicles, as a trial, was ridiculous. Stewart Joy, Beeching's Chief Economist, says so in his autobiography.
BR had two design centres for its dmus, Derby for the suburban and local lines designs and Swindon for the cross-country and intercity models.
Plus of course Eastleigh, who did their own thing and put diesel-electric power units into emu bodies.