Under TMA70/S108, for the purposes of company taxation, a company must act through its proper officer or, except where the company is in liquidation, through a person who at the time has the express, implied or apparent authority of the company to act on its behalf. You can serve documents on a company by serving them on the proper officer. This is, however, without prejudice to the charge and assessment of tax on non-resident companies (see AP164 and the second subparagraph of AC2050).
The proper officer of the company or any other person authorised by the company should sign the declaration in a tax return or amended return (see CTM93170 for more on this aspect).