Which device are you using? WICED Sense Board?
this is actually our own pcb design centered on bcm20737s.
First off, you are referring to the HCI UART correct, not the peripheral UART (there are 2 on the part, HCI is dedicated to programming the part).
SIP Module not SoC?
If you look here at the TAG3 schematic [Hardware User Guide (SDK 2.x and TAG3 Board)], it uses the SoC and the HCI Rx/Tx lines are mapped to physical pins 12/13. On the SIP module, these pins are 18/19 respectively (BCM20737S Bluetooth Low Energy SiP Module Technical Reference)
This is easier to see on the WICED Sense schematic as it uses the SIP module, not the SOC like the TAG3 board: WICED Sense Reference Design Schematic (PDF)
The HCI UART pins cannot be remapped.
The HCI UART is pretty tricky to get working the first time through on a custom design as the state of the port needs to be initialized in a specific manner in order for the part/firmware to come up in programmming mode.
In this thread, I outline several of the specific issues customers typically run into: How do I program a BCM20736?
However, the PUART can be remapped on a custom design, so it's important to keep in mind that the GPIO is physically mapped to a BCM2073X device within the platform.h file which is custom to the TAG development board by default.
#define GPIO_PIN_UART_TX 32 (Physical Pin 25)
#define GPIO_PIN_UART_RX 33 (Physical Pin 24)
This mapping in platform.h should be reconfigured for a custom design.
Note that the values in platform.h then map to the definitions and function prototypes in bleprofile.h