Hi,
I have checked the schematics, the P2_10 (pin53 of LQFP) is no-connection(float) in your schematics. You should have connected the P2_10 to a button so that you can control it's logic low or high after Reset.
During/after reset, if the P2_10 is sampled low, the ISP mode is entered, you can download application code to flash of LPC1768 via uart. If the P2_10 is sampled high, the LPC will execute the application in flash.
As a workaround, you can connect the P2_10 to GND or VDD via a flying cable so that you can control the logic.
Pls refer to section 32.3 Description in UM10360.pdf.

Hope it can help you
BR
XiangJun Rong