Apologies for the delay in getting back to you. Once the main clock has been enabled by the hardware, the peripheral clocks to UART and SPI are enabled hence why you were able to use the UART function without explicitly enabling it in your code first. This is explained in more detail in section 7.2.5 of the D2000 Datasheet.
Regarding the query about how much current is drawn when UART is enabled, this very much depends on the design of your platform which is using the UART interface on the D2000. Is you query related to the MCU silicon itelf, or the overall board platform? The silicon figures we have would not take into account IO considerations etc.