Your memory is sound, virginblue.
Air Southwest did fly from PLH and NQY to BRS where, several times a day, the aircraft would be usually parked next to each other to enable transfers to/from MAN and LBA the final destinations of the aircraft.
The BRS to MAN and LBA sectors were quite popular according to CAA stats with around 30,000 (36,000 one year) travelling between BRS and both MAN and LBA in 2007 and 2008. 2009 saw a decline in passenger numbers which carried on over the next two years after which the routes were discontinued (and PLH was closed anyway).
The recession probably played some part but the change of franchise from Virgin Cross Country to Arriva Cross Country trains on the South West/Midlands/North/Scotland route at the end of 2007 also probably had an influence, especially to Manchester.
With Virgin it was usually necessary to change trains on the Bristol-Manchester route. With the introduction of hourly direct trains between Bristol and Manchester with a journey time (as you pointed out) of around three hours the train became more attractive
Bristol-Leeds also has an hourly direct train service with a journey time of a fraction over three and a half hours.
Whilst accepting that there are some who would welcome the reinstatement of an air link - some members have recently pointed out the less than perfect train service - I really have considerable doubts that there would be enough takers to make an air route viable which seems to be the view of bmi regional.