Thanks Ceretrea
I guess the RSPCA could get round the issue by having "grades" of housing. A recommended minimum size for long term housing for pet owners, could be one, with an acknowledgement that there are circumstances when a smaller cage may be necessary for health or age reasons, but no smaller than xyz. An absolute minimum for rescues or pet shops, as temporary housing, could be another grade or level, but with the proviso that bigger would be better.
The ASPCA just say "no smaller than" a 10 gallon tank, which most people feel is too small for a long term cage.