Surveillance is used because it's about the ongoing collection of data and information. As in you closely observe something. It's widely used in Pandemic terminology.Yeah "surveillance" is certainly a strange word to use, but I suspect it's just a case of the writer couldn't think of a better word/phrase to use at the time.
The 1205 figure yesterday was from those that had tested positive. We know the actual figure will be a lot higher because not everybody with symptoms will have tests and some will be asymptomatic. 5 times higher seems reasonable and in line with ONS produced figures. maybe they are quoting the ONS figures.
It's not new terminology really.

