As Jokobie mentioned, doing a compression test or leakdown will help diagnose it before pulling the head. You can borrow tools from Autozone, etc that allow you to pressurize the radiator and see if the system pressure drops gradually - telling you there's a leak somewhere.
250ml daily is a lot of coolant and signifies something is wrong, but check those tests for peace of mind before doing anything. If you do end up pulling the heads, you can do a visual check for cracks yourself before sending it in for testing. The cracks most often occur from an exhaust seat to the water jacket closest to it. Sometime between an exhaust and intake seat, but it's rare. Hopefully that helps diagnose the problem, but do get it looked at soon because the temp gauge isn't much warning before it's too late.
Best of luck!