From next year, since First will run all the services around here, how about the following idea for train services

London to Cardiff like normal but at Swindon, divert onto the Melksham Branch calling at Melksham and Trowbridge, then back along the line to Bath and Temple Meads
London / Cardiff to Southampton again up to Swindon / Chippenham but then using the Melksham line calling at Melksham and Trowbridge then following the Wessex line down.

Im not sure of the maximum allowed cpacity and frequency on the branch but it could be workable

I cant see too much logic in a London to Cardiff service via Melksham and Trowbridge.  I cant see anyone using that service to get to Bristol and Cardiff as they would have a much longer journey, also can the signals around Trowbridge allow reversable working?
London to Southampton via Melksham sound like a good idea although would first be allowed to run such a service as London to Southampton is covered by South West Trains from Waterloo. (It takes about 1 hour 15 from Waterloo to Southampton, about the same as Paddington to Chippenham)  Another problem with such a service would be line capacity - would there be enough especially on the London to Swindon leg of the journery?
Graham I think you are right only one train can be on the line between Thingly and Bradford junctions