The 323s were procured by the Regional Railways sector of British Rail and built by Hunslet of Leeds (the only recent passenger trains for the UK main line network and I think the last before they went out of business). They have some stylistic similarities to the Networker fleet which was built a few years earlier by BREL and GEC (now parts of Bombardier and Alstom) for the Network SouthEast sector. Traction packages for the 323s were by Holec of the Netherlands, also their only product for the UK, also now part of Alstom.
One common factor is that 323s and Networkers have the same bogies. The design started with the PEP units and Class 313 in the mid-70s and was used on all multiplie units after that until privatisation except Class 158. It was a BR design and although the rights passed to BREL/Adtranz when that was privatised, they were required to license the design for any stock ordered by BR from any supplier. Hence it could be fitted to the GEC Networkers and the Class 323, but after privatisation when BR ceased to exist it only appeared on Bombardier designs.