First, note that the Underground already uses regenerative braking. This article is about a new generation of braking technology.

The Gizmodo article at http://gizmodo.com/london-underground-is-trialling-regenerat... is better than this BBC article. (And the similarities help show which parts come from the press announcement.) It also mentions a £6m/$9m per anum power savings across the entire system.

Further, the article at The Memo https://www.thememo.com/2015/09/25/regenerative-braking-tech... says "If rolled out, Transport for London (TfL) estimates that the technology could save £6m every year in energy costs, or 5% of its total energy bill."

The Wikipedia article at https://en.wikipedia.org/wiki/London_Underground_cooling says 'About 80% of the heat comes from the operation of the trains, 15% from other equipment, and 5% from people'.

Putting it all together, the extra savings (5% of 600 GW-years) is equivalent to about 1/2 of the heat generated by the passengers.

That's about all can figure out on the topic.