Sorry didnt realise Trowbridge was a busy station. I knew Westbury was an important one as people connect into many other services there. There are many other trains which call at Filton tho so people could connect at Bristol Temple Meads. This would provide a non stop service from Cardiff/Newport to Bristol TM‡.
Could there be scope for a limited stop between Southampton to Portsmouth and Southsea? What other TOC▸ run services along this line?
I think a limited stop Cardiff-Portsmouth service returning to what it was a few years ago would be a good idea provided good stopping services are provided in between the fast services. I find it surprising there isn't demand for 1st class particularly between Bath/Bristol-Cardiff.
In answer to your question, the other TOCs operating on the line between Southampton and Portsmouth are
SWT▸ and Southern (not all the way to Portsmouth)