Hello,
I think this question goes beyond the scope of emWin like that. It is more about making appropriate drivers which will work with external flash (storing, reading data...) and making options in your linker files for placing data at corresponding sections within the flash. Since you use LPC you better should contact NXP forum.
That way works - i had done this on a STM32F746 and Flash. The way is not different from protected bootloader.
You have to build a structure that contains pointer to the memory start of every font (or others) your store into the flash. The start of the structure can be stored by the linker on a fixed memory address. In the program code, you point with teh strutre to the start address of the flash memory where the structure stores.
I had seen a different methode, where every object like a font, Picture or ever, stored in an ROM, marked by a unique lable at the start address. Inthe programm memory , a routine search the relevant memory range the symboles and store the address into a structure pointer. This methode is flexible, but need startup time to find all symbols. A varius version of them i had used to find symbols in a protected bios to find the init entry point of API structures for upcalls.
Some things have to be more care like IRQ or DMA processes. I had enought memory and copy the used objects into the RAM. For large objects like pictures or databases, it's required to declare the flash memory as Read-Only in the linker file / ld-file or memory-map; depended on the toolchain you prefere. That's important that you do this exact. In the GNU toolchain, that's a worse job. I hate ld-scripts, even they are powerfull. In SES, that's a quite easy job. Hope this will helps you to find a solution for you.
BTW: I had found an example for the STM32F746Disco and the STm32H747 Disco board. ST distribute a demonstration how flexible and powerfull the both MCU and Boards are. This are available from ST, and they load up every static objects into the external QSPI Flash and load them at startup. The source is free and it could be easy mirated into SES by using an extern toolchain Build.
The source is not pretty writen like them from Segger. But for just an example and inspiration that's could be helpfull.
Good luck