Low-level signals are small repeater lights that mirror what is displayed by the larger, conventional, traffic signals at junctions. They make it easier for people cycling to see signals that apply to them, without having to crane their necks back.
As in the picture above, they are used in combination with these large signals in the Netherlands.
The role that low-level signals play is simply one of convenience, rather than some radically different way of signalising junctions. They make it easier to see what the signals are displaying, rather than being a stand-alone signal in their own right.
The high level main signal should remain a requirement, principally because it is easier to see from a distance than a low level signal. Without it, people may have to slow and/or stop unnecessarily, because they can't see a green signal until they arrive at a junction.
In 2013/14 low-level signals were employed at a number of locations in London, in on-street trials. They are employed in Britain in precisely the same way as they are in the Netherlands - mirroring a conventional signal, which must always be present.
Low-level signals are included in the 2014 consultation on proposed changes to the TSRGD.