The problem with buskers at stations isn't that they exist but that there is no guiding artistic philosophy. TfL should have an artistic director who schedules and promotes jazz, punk, and grunge musicians at the right spots in the right stations at the right time.
@anon_opin that sounds like an excellent idea, although you'd also have to ask the buskers what they want out of their business model, it would suck if scheduling were used as a cudgel to force buskers out of the stations, ruining their livelihood, in favor of less needful acts. Maybe have the _buskers_ organize the calendar, if they don't already have some way to deconflict where they play.
And of course, I would think that any act scheduled should also get a stipend to make it worth their while, in addition to whatever revenue they can earn busking.