What real difference would a short DMU▸ running a handful of services a day through to Waterloo actually make in the grand scheme of things? Not much I would suspect.
Indeed - but then an hourly through service all stations Bristol Temple Meads to Salisbury ... and onwards all stations to Basigntoke, Farnbourough, Woking, Clapham Junction and Waterloo ...
would make a difference.
"Where would the trains and crews come from?". Well - the Bristol to Westbury is already there. In many hours that extends to Salisbury already, and in many of those it doesn't it carries on to Salisbury. Salisbury to Waterloo is already there. So it's a bit of joining up that's needed and NOT a handful of (extra) services that need to be found from somewhere.
I do accept that at times that the line into Paddington is not available, but Westbury IS, the though Bristol to London (Waterloo) service would be distinctly busy. I'm not sure where we're going with 158s, 159s, 165s, 166s and 175s next year - but it's not beyond imagination to suggest that 158s could be replaced on Cardiff - Exeter services by
IETs▸ not headed to London on those days, with the 158s used to make the Bristol - Salisbury services up to 5 carriages, with 8 onwards to Waterloo.