I can't see why you would need to say where you access the information - it's not really relevant.
I must also say that I don't like the Harvard System - it can be so cumbersome
The TNA reference in itself is only informative to the initiated, the uninitiated need to know what information you are citing and its date.
I would suggest:
Census (1861) The National Archives (TNA): PRO RG9
subsequent references:
Census (1871) TNA: PRO RG10
In the text you would put something like:
Sheila was a lace maker prior to volunteering for nursing service [Census 1891:1234 f1 p2] ...
Or according to the census [1901 f34 p21] Betty was living in ...
Horrible, but Harvard
In theory you should perhaps use UK Census to distinguish it, but I guess the context is such that it would be self-evident. Anything to make it less cumbersome.