Basically there are a limited number of ways that you can take money out of a limited company in which you have shares. These are:
a) by paying a salary on which you pay Income Tax and NIC
b) by paying a dividend out of the companies distributable profits
c) by repaying a loan account i e. where you have loaned the company money.
If you do take money out as repayment of the directors loan account, you must be careful not to go overdrawn by more than £5,000 at any time during the tax year. Otherwise it will have implications for your taxation because of the deemed benefit as well as having corporation tax implications (loans to participators).
Alternatively you can create the new start-up business as a limited company and make the existing company a shareholder. If your existing company has other shareholders apart from you, you must seek their consent and obviously it would mean that they have an interest in your new start-up business. They may want a say in the running of the new company and this might not be what you intend.
If the new start-up business is in any way risky it might be better to limit the cash injection and the interest of others until it is a demonstrable success.
A discussion with your accountant should help to decide a way forward.