Being a member of the RIBA does not entitle you to call yourself an architect. That is governed by law and the ARB and, as Justin says the reason for this liability.
The chances of the British Government ever protecting the title of 'architectural visualiser' or similar is less than zero, they would not consider protecting the titles of Engineer, Surveyor, Contractor, Builder, Clerk of Works etc. (despite extensive lobbying from some massive engineering institutions with many thousands of members) so protecting the consumer against a badly rendered perspective would be given pretty low priority.
The list of protected titles in Britain is very small indeed (much less than the US), it includes Solicitor and several health related titles - doctor, osteopath etc. but not much else.