LPC2148 baud rate equation
The only thing that makes sense is that the equation in the datasheet has an error.

That same datasheet says:


  The U0DLL and U0DLM registers together form a 16 bit divisor where U0DLL contains the lower 8 bits of the divisor and U0DLM contains the higher 8 bits of the divisor


That suggests it should be 256*DLM+DLL as written in tutorials. In example tables below, it's also treated as 16-bits altogether (upper 8 + lower 8).

Note that April 2012 version of datasheet [1] contains the corrected equation.

[1] https://www.nxp.com/docs/en/user-guide/UM10139.pdf