emailed the PLA and got this prompt reply
Thank you for your recent query regarding a new speed limit on the tidal Thames.
I can confirm that recent media reports relate to a temporary requirement for certain high-speed passenger vessels to adhere to a 12 knot speed limit in central London.
In the wider context, the PLA has, with the active involvement of vessel operators, recently undertaken a comprehensive formal risk assessment to identify what additional risk control measures we need to consider to ensure that the risks to public safety and general navigation posed by the operation of increasing numbers of high-speed vessels in central London are managed to 'as low as reasonably practicable'.
One such identified risk control measure is the implementation of a mandatory speed limit in central London.
As such, the PLA is currently drafting revisions to its Byelaws, which include such a mandatory speed limit.
In line with long established procedures, the PLA will be consulting widely among the port and River community on its draft Byelaw proposals. We hope to start the informal consultation process in the next month or so.
Details of the consultation and our proposals will be published on the PLA website in due course.
Harbour Master (Safety Management System)
Port of London Authority
Tel: 01474 562266