I think most people would interpret that sentence as meaning that several thousand lives are being saved every year by communication data requests made by the police. But really you would need to know the total number of these communication data requests that are being made on average as part of a specific police investigation.
The following document throws a bit of light on it:
The relevant bit looks like the following paragraphs:
"Home Office published statistics show that there are in excess of 4 million crimes reported annually and around 1.4 million of these will fall into the serious crime category. Criminals will often use many communication devices at any one time and will regularly change them. A significant murder or organised crime investigation can involve up to 500 communications data requests or more often mainly subscriber checks.
Communications data helps focus an investigation by identifying possible suspects, and is also critical in confirming alibis, ruling people out of further enquiries, and finding witnesses. Many tens of thousands of communications data requests are made every year in urgent threat to life situations: e.g. to find a vulnerable or missing person or in kidnap situations."
From the above it looks like there could be as many as 500 communication data requests per investigation by the police, so the 7500 to 12000 figure would actually correspond to a much smaller number of lives being saved.