I don't see why not.
I have done this myself especially in a small garage, conservatory or outside light where the existing CU was a 16th ed. (no RCD on lighting) split board, and used a SFCU for the lighting.
I was once pulled by the LABC inspector, who insisted that I used 1.5mm for the lighting circuit, in case someone replaced the 3 or 5 amp fuse with a 13A, so usually do this as a matter of course when using this method.
You need to make sure it is labelled and noted.
I would like to point out this method is not my preferred way of doing things, but can be used to get you out of a jam, or if the customer is unwilling to disturb existing decor etc..