British Midland needed the extra capacity after European route licence “wins” - I think it was the Heathrow-Amsterdam licence - and put the 1-11 on Leeds Bradford-Heathrow as the shortest sector where the 1-11’s fuel burn versus the DC9 could do least damage (or so it was said!).
I don’t know if BMA had any choice but there were a few occasions when G-WLAD went tech and G-YMRU was sent in to take over and also went tech at LBA.