All entirely possible.
Most lenders do not pose a minimum mortgage level to take out.
A mortgage must be for at least £25,000 for it to be regulated under FSA regulation. if it is any less than thuis then it falls under the consumer credit act regualtion....it is no less legal a loan and if you miss payments you gonna be just as ***. There is no minimum loan you can take and secure against your home but some lenders may have restricted ranges and some may not offer this type of lending but there will still be plenty of very competitive products
My suggestion would be to look for a lender that will charge no fees - No arrangement or booking fee, valuation fees and legal fees and then see what they can offer to you as a remortgage for £15,000. There will be lenders that will do this but they may charge a slightly higher interest rate to do so. Don't worry about this as the interest rate is the least of your worries on a loan so low. it is far more important to ensure that the fees are as little as possible.
You can get fee free products from around 3% paying around £145 on a 10 year mortgage term but if you went for the best rate product you could get a rate for around 2.25% but still be paying £139 but also have a £1995 fee to take on top of this, add the fee to the loan and it becomes £158 per month.
Yes you can do what you want, just arrange the loan for the value you want and then before the new mortgage completes you will be asked for the difference between the old current balance and the new proposed balance. pay this to the soklicitor or directly to your lender and your new mortgage will be based on the lower balance.
The other choice would be to either stay on the variable rate and pay of the difference but you may not get quite such a competitive deal.....but it would be far less hassle or you could apply to another lender for their variable rate mortgage meaning you could pay off any amount at any time you wanted to....however i cannot be sure which lenders may do this on a fee free basis.
if you have any doubts or worries then go and speak to a mortgage broker and discuss options with him