Avanti West Coast have announced today that they will now issue through Rail Tickets to Keswick via the bus from Penrith for a £2 add on each way- the same as the bus fare.
So it saves nothing, it is purely for convenience.
Whether it is worth doing depends on your plans when in the Lake District. If using the bus while there to other places then buying a Stagecoach weekly megarider from the bus driver at Penrith may still be the best solution.
But if you are not straying outside Keswick it will be a worthwhile thing to do. Also if the bus is late arriving at Penrith then even on an Advance Ticket you can take the next train. Likewise on a late evening train- if you miss the last bus from Penrith Avanti have to provide a taxi to Keswick.
It is not clear when this is starting, but presumably when it does Keswick will appear as a 'station' on rail ticket planners.
EDIT- I have checked this morning. This scheme has now started and is valid on all operators including Trans Pennine Express to Manchester Airport. So Keswick to Manchester Airport is as low as £12 (under £8 with a railcard).
There are two destinations in the system for Keswick-
Keswick Bus- Station Code- QKD- this is a non functioning dummy code;
Keswick Bus Station- Station Code - KWK- this is the one you want.