Maxsteam is right that you cannot guarantee what your repayments will be next year or the year after. You also have to factor in that you have a penalty to pay until next year which will likely add another £1200 to your costs but based on a simple calculation I estimate that based on current rates available you could reduce your mortgage to around 8 years duration and still be paying £448 per month (give or take).
The benefits of paying the money off the mortgage now would be that once it has been paid then it cannot have any further interest charged against it at what ever the rate will be, this can often save you thousands of pounds on your total cost. In your case I think I worked it out to around £9,000 savings on interest payments by paying off the £25,000 now and reducing term by 5 years. (based on current rates)
Investment could help but the rates available for this are quite poor at present and while you may make an eventual return on your inverstment it will be offset by the interest that you will have paid on your current mortgage balance over the current term. Once the money is paid of the mortgage then it is paid off and will not have any further intertest charged against it.
To discuss your best options I would suggest that you go and speak to a financial adviser who can guide you through your options and will most likely be able to show you the whole of market mortgage options.
Be aware that if you are tied to your mortgage till 2013 that you WILL have to pay a penalty to your lender to be released from this contract and I would estimate that this would be at least 2% (£1200 approx) of your existing mortgage balance