The claim for under-investigation comes from the fact that in English law rape is a very serious offence, and carries a similar sentence structure as murder. There are 150,000 rapes per year (according to the annual crime survey, which probably under-counts rape) and there are only 5,000 prosecutions each year.
Here are the stats showing decline after the collapse of the high profile cases: https://www.ons.gov.uk/peoplepopulationandcommunity/crimeand...
Here are some news reports:
https://www.theguardian.com/society/2019/mar/06/prosecution-...
https://www.bbc.co.uk/news/uk-48095118
https://www.independent.co.uk/news/uk/crime/crime-statistics...
https://www.bbc.co.uk/news/uk-45650463