Currently the 0544 train will get you to Euston at 0907 for £182. To arrive into London at a similar time the 'free shuttle' would have to leave Carlisle at 0530 for a 0600 check in, a 0630 departure to connect with a train from SEN around 0800
Your example assumes both journeys start on the platform at Carlisle station?
The point made on the podcast was transit times through CAX would be similar to catching the train.