BKS and Cambrian got rolled together into an intermediate holding company, British Aviation Services, which BEA and then BA owned. BKS was named after its founders, Barnby, Keegan and Stevens, and after finally relinquishing ownership Keegan made issues and the company was renamed as Northeast, their general territory, although their initial base had been Southend. BAS didn't do a lot but did devise a common livery style, using yellow for Northeast and Orange for Cambrian, over a grey cheatline.
I think the crews were on different terms which accounted for them remaining independent entities. It was common to exchange aircraft among each other, and have BAS aircraft operating for BEA. One of the Cambrian One-Elevens was based in Berlin for about a year doing BEA routes.