When I worked at British Midland (and BA
were no different), we used to have BD1, BD51, BD81, BD101, BD121, BD221, BD581, BD771, etc, etc all inbound to LHR
first thing in the morning. From a commercial point of view it was and still is important to maintain the concept that the first flights into LHR
each day ended in a 1, and the next ended with a 3, then a 5, 7, etc. So the GLA
-LHRs were BD1, BD3, BD5, BD7, etc whilst AMS
were BD101, BD103, BD105, etc (the outbound flights had even numbers such as BD2, BD4, etc). Having this flight numbering framework makes it easy for the passengers to understand what flights they want, whether they travel on the same route all the time, or on different routes.
Operationally of course, having all these flight call signs ending with a 1 ("Midland one", "Midland 81", "Midland 101" etc) decending into LHR
at roughly the same time caused problems, confusion and on more than one occasion misinterpretted instructions. So, operationally a lot of flights had their call signs changed / renumbered (including letters) to remove all chance of confusion, whilst the commercial flight numbers that passengers use remain as they always were.