BIA could trace its history to its ancestors namely Jersey Airlines. Manx Airlines and Lancashire Aircraft Corporation. Silver City bought Manx and the northern operations of LAC to form Silver City Northern Division.
Air Holdings- partly owned by the Cayzers (British and Commonwealth) as a result of their ownership of Hunting Clan Airways which was one of the founders of British United Aiways- purchased Silver City and Jersey Airlines in 1962. The northern division became British United (Manx) Airways and Jersey Airlines became British United (CI) Airways. These two were run from Jersey by the old Jersey Airlines management. the route structure was very similar at that time to what it was in 1978.
British United (Manx) and British United (CI) became British United Island Airways on 1st November 1968 by which time British and Commonwealth had Bought the Main British United Airways From Air Holdings
When British and Commonwealth sold BUA to Caledonian in 1970 BUIA was not part of the deal and was renamed British Island Airways
Air Anglia was the result of the merger of several small East Anglian operators in 1971 British and commonwealth bought the company in 1978 and did announce at the time that they intended to run the two companies seperately but subsequently changed their minds and Air Uk was born in early 1980
The two route structures evolved entirely seperately and the BIA structure was much older and the two airlines were not working in cooperation until 1978