This depends on the loan’s maximum allowable “LTV” (loan-to-value ratio).
For example, if your house was worth £500,000, and you had £200,000 still owing on your mortgage, a homeowner loan specifying a maximum LTV of 80% could allow you to borrow an additional £200,000.
Each loan application is considered on its own merit though, so having sufficient equity doesn’t guarantee approval. But alongside being able to afford the repayment schedule, having sufficient equity gives you a much better chance of getting your application for a £200,000 loan across the line.