Apparently, it's to encourage cycling instead of public transport in these unprecedented times, creating a safe zone for cyclists.
The first one appeared on a short stretch of the A6 when lockdown pt 1 was being eased, and had a largely negative reaction from pretty much everyone on the local FB group, with comments such as: it leaves nowhere for drivers to go when an ambulance is flashing its way through the traffic (this is the final stretch to A&E). The lane isn't wide enough to allow one cyclist to overtake another. The cones are too close together, making it tricky to get out of the lane. The cones are designed to pop out when struck, which seems to happen a lot... there's always one laid somewhere in the lane.
Last week, the lanes were extended further, much to the bemusement of the local FB group. I reckon it's just another case of the non-cycling planners deciding what is best for cyclists without actually consulting them.