In a TN system you can use an RCD for fault protection under ADS in compliance with the minimum standards for installation as provided in BS7671.
However good practice would be to use all the available means of protection, which would include designing a circuit that can use both the RCD and the MCB for fault protection and so allows greater protection thatn the reliance on a single device.
This is a simple matter and does not cause any problems in design so why not do this.
Obviously in a TT system you do rely on the single RCD unless you can get TN levels of Ra, but this is costly and difficult to do in general domestic situations.
