There have been a few high profile cases, and, technically, it is correct
that one should obtain planning consent for change of use, before the
event,
but I wonder how many people have made such a change informally in the
past
and got away with it?

It undoubtedly happens all the time. That is not to say that it is wise to
do so. The difficulty arises now because of a recent court case where
unauthorised work had been done. As a result any prudent solicitor will
require written confirmation that all building regulation and planning
permissions have been granted. Making a false declaration can prove very
costly. If I recall correctly the case referred to cost the vendor some
£67,000 in compensation and costs.