Any invoices or contracts in the name of the company must be dated after the date of incorporation of the company.
Can I start work before the company is incorporated?
Yes, provided any invoices for the work you have done are dated after the date of incorporation.
Can I sign a contract before the company is incorporated?
No. You cannot enter into contracts in the name of a company before the company has been incorporated. However, you could check with Companies House that the company name is available and draw up a contract in the proposed name of the company, leaving the date to be inserted when the certificate of incorporation is received.