A secured personal loan is one in which some of your property is held as security by the lender for the amount you have borrowed. Secured loans usually offer lower interest rates than unsecured ones, and are often the cheapest way to borrow.
If you have kept up to date on your mortgage repayments since you took out your original mortgage your bank or building society lender will be very keen to lend you more money. You should be able to get the increase on the same terms as your existing mortgage.