The Companies Acts 2006 sets out the law on the running of companies. The 2006 Act adopts the Model Articles of Association which set out how the company is to be run on a day to day basis by the Directors and shareholders. The Articles will cover such things as the directors, decision making, shares and distributions and meetings. Your solicitor can advise you further as to what you may need to include.
The difference between the articles of Association and the Shareholders Agreement is that the latter is a private document between the parties whereas the former is a public document filed at Companies House. Matters which the parties do not wish to make public and which are to remain confidential may be covered in the Shareholders Agreement.
Speak to a company law solicitor today who will be able to help you with expert advice.