There are over 18 companies* supplying electricity in the UK at the moment. Whether another one joining the throng will change the game, who knows? No doubt the idea of Bro-Ken's that he would buy electricity through TfL if elected has either struck a chord with the public or struck 'terror' in the heart of the Conservatives, again, who knows?
All in all this is just more tinkering at the edges. The big money is being made within the Big-Six through their business models of buying gas from 'themselves' and charging 'themselves' whatever amount suits the moment. The more electricity generated through gas bought on the [cruel joke alert!] 'open market' the less difference which supplier you use makes.
*
http://www.electricity-guide.org.uk/companies.html