If you wish to register a non-trading limited company, you must set up a company limited by shares or guarantee at Companies House and then notify HMRC as soon as possible after incorporation that your company is non-trading. It’s exactly the same as setting up a company that you plan to trade through - the trading status aspect is dealt with by HMRC after company formation.
A non-trading company does not carry on normal business activities like selling goods or services, but it still has financial transactions that need to be entered into its accounting records. These transactions could be things like paying rent on business premises, employing and training staff for when the business starts trading, paying accountancy or legal fees, etc.
To notify HMRC about your company’s trading status, you must get in touch with your local Corporation Tax office - their contact info can be found on the letter that HMRC will send to your registered office soon after incorporation. You can also contact them here.