Remortgaging would probably involve fees and legal expenses so I doubt if it would be cost effective.
Your bank should provide you with an overdraft or term loan if you have a good credit rating. €10k @10% for three months will only cost €250 in interest, so shopping around won't save you that much.
CUs are not necessarily the most competitive for loans especially when the cost of keeping money on deposit/in shares (which they normally require) while borrowing is factored in. Many other financial institutions don't impose penalties for early redemption of loans (other than fixed rate loans). This is not unique to the CU.