For nearly 30 years, the UK's wiretapping laws have been the subject of annual reports. Since 2002, they are available around the web (for now), but earlier than that, it is a rabbit warren of possible locations.
In practice, the reports are solely available from the Parliamentary Archives if and only if you are a member of an institution which has paid for access. Requesting a copy from elsewhere sends you to this destination.
The current Interception of Communications Commissioner didn't particularly want them on his website, as many agencies are (and were for years) paranoid about people knowing how many warrants are in use at any one time. As much as we disagree with his opinion that a historical representation of oversight isn't important, we disagree more with his belief current oversight is effective.
So when the Government claims that these reports provide public evidence of transparency and oversight, those claims are meaningless if the historical reports can not be read.