These are Home Equity Loans. With these, you borrow against the equity in your home. They will usually only offer up to 75-80% of equity. That means in your example, if a lender offered 80%, the math would look like this
80% of £150,000 = £120,000
Amount owed £100,000
£120,000-£100,000 = £20,000
In this scenario, you may qualify to take out £20,000. If you are considering doing this, contact banks and ask them about equity loans.
It would be a separate loan that you would make payments on. You still owe your primary mortgage.
Home Equity Loans are usually for shorter periods than mortgages. I believe that both of mine are for 10 years.