To have your company struck off the register, you need to send form DS01 which can be obtained from Companies House. The form must be signed by a majority of the company’s directors. If applying via post, you need to also send a cheque for the appropriate fee which is currently £8.
You can also complete the process online through the Companies House website.
HMRC’s resource is helpful in being able to provide further information on the responsibilities of closing down a company correctly, when a company cannot apply to be struck off, withdrawing a strike off application and more.