It has been announced in the press that there would be 9000 jobs in total created in the construction industry for the Thames Tideway Tunnel - "Supersewer wins business and union backing". There is a question on the duration and local / UK potential for these jobs. Any jobs created by large infrastructure projects needs to be compared with alternative solutions.
The BG strategy adopted by Philadeplhia has already generated a significant industry in the metropolitan region with more than 2,400 firms, 32,000 employees, and in excess of $7.4 billion in sales. See page 9 of report "Capturing the Storm: Profits, Jobs, and Training in Philadelphia’s Stormwater Industry".
A "stormwater industry" can be developed into a wider "Blue Green industry".
Similar studies need to be done for London which is many times larger than Philadelphia. A conservative estimate (to be verified by appropriate study) might lead to the generation of over 100,000 jobs which may be predominantly low skilled and local.