The Herald was better suited to the freight role, the large rear doors and robust interior were an advantage over the F27. Towards the end of the Herald's working life the shortage of spares and lack of manufacturers support made maintaining the type rather challenging. I was one of the BIA engineers at Blackpool that moved down to Norwich on the formation of Air Uk. I spent around 20 years as a type rated LAE on both aircraft types, the F27 was a doddle to maintain, but I have very fond memories of the Herald despite the problems.
In the main the Herald operation was reasonably accident free with Channel Express, but we did have several bad incidents with the F27.
Back to the original OP question, two factors that I recall that may have prompted the parent Company to buy Air Anglia may have been the east coast routes and definitely the hangars and support shops at Norwich were far better than the old sheds at Blackpool.