A lot of entry deals and low fixed interest 'honeymoon' periods attract an early redemption charge if you pay the loan off in an earlier time period than what your loan was originally taken out for.
These charges allow the lender the chance to recoup the forgone costs that would've been applied if the mortgage was a straight up variable loan and ran its due course.
Usually, the charge is a percentage of the loan you are repaying, or a number of months interest. Most charges are payable only during the special offer period, but in some cases they are levied beyond that - these are called overhanging redemption charges.
Now to decide if it's worth it to pay out the Early redemption charge then you must ask your lender for a pay out sum of all charges, including any mortgage release fees.
Sometimes it may be worth it to wait until enough of the loan is payed before getting out. Other times if the interest is highly based for years to come, paying any early redemption fee and remortgaging with a new lender makes good sense.