It is possible to convert your loan into a capital-and-interest mortgage and borrow extra for the purposes you mention, but only if your documented income supports the total loan and there's enough equity in your home.
As mercman says, it may not be the best time to cash in your endowment policy - this requires investigation. Your options are (a) to keep paying it to maturity alonside your new mortgage, (b) to cash it in now and reduce the amount you need to borrow or (c) to make it paid-up, i.e. stop payments but don't cash it in at this time.