When a director transfers money or injects cash to his/her business, s/he effectively loaned money to the business which they can take back tax free later when the company generates revenue. This is called a Director's Loan.
This is what you did already.
However HO requires you to to prepare a specific loan agreement which makes your loan to be unsecured & subordinated to 3rd party investors.
This simply means that if you borrow money from other sources (credit card, other loans, ... etc) , you MUST pay ALL those debts first before you can take your money back. Also in the event of bankruptcy or insolvency, you are also forced to pay all the creditors first if your company has any liquid asset and the debt to you will be the LAST to be paid back if any money is left.
You you need to prepare a loan agreement as required in the guide, provide business bank statements showing you transferred the money to the company and submit accounts showing that there is director's loan under your name. The guide is very clear on this
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