Hello, I hope I can offer you some assistance.
As the battery is new and measures at 12.5 volts when stood then the fault may well be a starter motor concern. If even with a jump start the engine is slow to turn over then before replacing the starter motor check the wiring to the starter motor. Use a multimeter to measure the volt drop between the positive terminal of the battery and the main lead connection on the starter motor when trying to start the car, the reading should be less than 0.5 volts, any higher may indicate a high resistance in the wiring loom. Do the same on the earth side of the starter motor, measure the volt drop between the starter body and the negative terminal on the battery again this should be less than 0.5 volts any higher check the main earth lead for the engine.
If both readings are OK then the fault will be the starter motor.
Let me know if you require further assistance and I'll reply as soon as I'm able to.