I think you are being pretty harsh on yourself in terms of your level of experience, you do have real evidence of your achievements in industry and no employer is expecting you to know everything on day one, just the ability to learn and work as part of a wider team:
. 10/0,4kV electrical distribution system for district area which has 22 building plots
. 10/0,4kV electric supply system for heating systems manufacturers factory facility
. Internal lighting design for manufacturers facility mentioned above
This is great as it provides you subjects to discuss in any interview or mention on any application form.
Have you considered that many a building services engineer works in the data center design - and targeted companies / consultancies involved in delivering those projects around the UK and internationally?
The IET covers a broad church of engineers, I am an electrical power engineer by trade, there is another institution who may be able to help in a more specialized manner:
The Chartered Institute of Building Services Engineers http://www.cibse.org/
I would certainly look at this section of their website if you have not already http://www1.hays.co.uk/jobs/cibse/jobseeker.html
They also have a young engineers network who may be able to help http://www.cibse.org/index.cfm?go=page.view&item=700
IET Cambridge Local Network website: http://mycommunity.theiet.org/communities/home/229
There is an event in June which may help you over in Cambridge: http://www.theiet.org/events/local/175084.cfm
Hope this helps for now. Ian