London is a big city. It is not easy to find someone but if you know his name then you can search on GOOGLE or FACEBOOK maybe these websites could help you.

Hi,
If you are looking for a specific person in the company, try to look for him/her on "Linkedin".
If it's the contractor as company that you are looking for, try to look online on the company's website and sending an email asking about the specific person, look for it on linkedin, or search in the list of building contractors based in London UK. Depending on the size of the company, you may be able to find it quite easily.
If you need help, feel free to contact me. I'm based in London.
Best,
Fabio 

thank you Atul!