The BBCs introduction of 50fps seems to have adversley affected --tvmode choices because it now incorporates a mixture of resolution and frame rates.
I think it would be helpful to rework/rename the --tvmode shortcuts so they reflect both the resolution and frame rate, as follows:
Best50=1280x720/50fps  Better50=960x540/50fps  Good50=704x396/50fps  Best25=1280x720/25fps  Better25=960x540/25fps  Good25=704x396/25fps  Worst25=512x288/25fps

Sorry, no. The "50" variants are essentially the defaults, there is no "Best25", "Better25" and "Good25" can be obtained via --fps25, "Worst25" is redundant.