Where will your London hotel be?
If I were doing the Torquay to London trip I would prefer GWR into Paddington. It will be faster with fewer stops along the way.
The split fare doesn't help in this case. There are some quite cheap fares through the day, and you can get on the train at Torquay and take a train which takes you all the way into Paddington without changing trains.
For example on the 22nd of October, in second class, the 07:55 service will get you into Paddington at 11:39 for £31.50 (3H44), or half an hour faster at 11:38, Paddington 14:54 (2:54pm) for £43.00 (3H16), or even faster at 15:00 (3pm), Paddington 18:08 (6:08 pm) for £31.50 (only 3H08).
There are numerous faster trains all day if you are happy to change trains at Newton Abbott or Exeter St Davids, so you could knock off half an hour or so, but I bet that it would be nice to settle into your seat and not have to change trains a few minutes after boarding? Those slightly faster trains are more expensive, too (£56.50 or £43.00).