In order to complete any work in the UK you need to be issued a Tier 2 work permit.
What you Need for a UK Work Permit Application
There are numerous requirements for getting a UK Tier 2 work permit. Firstly, you need a certificate of sponsorship from the employer you intend to work for (if you’re from outside the EU/EEA and not a Swiss national). This ties you to that employer for the duration of your stay in the country, and makes them essentially responsible for your presence. You have to get this sponsorship within three months of you sending your application. There are also several different documents you’ll need to send, including information about your qualifications and information about your prospective employer.
The application form (which should only be used for applicants who are ALREADY in the UK) is here:
Here is the form for dependent applications:
If you are outside the UK you need to check the information for your current country of residence here http://www.ukba.homeoffice.gov.uk/countries/
*Normally your employer or their registered immigration attorney will complete the form for you.