I think alexan_e answered much more than you asked.
Now it's time for you to become familiar with the very basic LPC2148 system properties.
After reviewing the mentioned documents, you'll realize that you are dealing with at least three different clock frequencies:
- Fosc, the external crystal or oscillator frequency
- CCLK, the processor clock frequency, PLL generated in the range of 10 to 60 MHz
- PCLK, the peripheral clock frequency, divided CCLK, drives e.g. the UART
Instead of repeating 12 MHz like a mantra, find out what's the actual PCLK of your design.