If you form a partnership to carry on a business, you must register it separately with HM Revenue & Customs even if the partners have previously been self employed. If the partners are new to business, they must also register individually. Melior will help you with this.
Partners in firms are taxed on their share of the profits of the firm for the tax year, and the basis of tax is similar to that for the self employed. Each partner is effectively taxed as if he were a self employed business, with profits equal to his share of the profits of the firm. So, instead of tax (and national insurance) being deducted from your earnings at source, you must be prepared to receive a bill at some time in the future.
There is a partnership return which shows the profit and loss account of the partnership and any adjustments made for tax, including the capital allowances. This shows the profit shares allocated to each partner on the Partnership Statement. Each partner then also completes a return on his own behalf showing the share of partnership profits corresponding to the partnership return.