During 2001 and into 2002 I used BHX-NCL service with BA almost every week up on Tuesday and down on Friday. It was 4x 1-5 and 1X 7 late afternoon, hot breakfast and bar trolley other flights. Weekend leisure fare, stay Sat night either way was £49 rtn. Of course in those days BA operated a hub at BHX on many times we were 'held' to await connecting pax arriving on BA (eg MXP, FRA, DUS CDG BCN and so on) and even AA ORD too all using the Eurohub terminal. At one point they dropped down to 3x 1-5 adding an additional NCL-CWL rotation instead. However after a couple of months this was swapped back. As a regular traveller I got to know the cabin crew and got all the inside info, CWL just didn't attract sufficient pax v fourth BHX service. I was travelling on business, although after this assignment I continued to use the service for personal weekend visits to wonderful Newcastle.
I very rarely used rail, to be comfortable on a long journey, in my case continuing on the same train to Leamington Spa, you needed to travel first class which was expensive, similar to unrestricted weekday airfare, and in those days the food service on the train was not good, and BA terraces lounges were a great place to wait if you got to the airport early.
Maybe the BA feed ability helped sustain loads and yield, the early morning flight BHX-NCL was always very well used as was the NCL-BHX evening return. I used the two other flights rarely but they were about 50% I recall.
Not sure this helps the debate but just wanted to share my experience from a while ago.