Re-mortgaging is probably not worth it if you have only small outstanding loan. You are unlikely to make much of a saving, or one that will drive down your secured debt loan to any great extent as fees can eat up any gains.
You should also look at early repayment charges, which could make it prohibitive to move in certain circumstances, for example, if you are changing mortgages well ahead of the end of an incentive deal that you are currently on.
If the value of your home has depreciated, then you are not going to be able to realise any gains. Similarly, if your circumstances are now different from when you took out your mortgage, you may not be able to benefit from re-mortgaging as you may not meet lending criteria.
Credit problems will also rule out a potentially lucrative mortgage switch.