The article seems to be referring to documents that have been released under the Public Records Act. Until 2013, government departments were to release most records to the National Archives 30 years after their creation (though there are exemptions relating to foreign relations or national security.) In 2013, the release deadline was reduced to 20 years, and since then the National Archives have been playing catch-up on records from the 1980s.

The National Archives seem to have a number of collections of Foreign & Commonwealth Office documents relating to the Chinese student protests in 1989, which I would expect to include a lot of material relating to the Tiananmen Square massacre. None of those releases appear to have been digitised yet, so the only way to view them is to visit the National Archives in person at Kew.

Fortunately, the original story your link is summarising includes legible photos of the Foreign Office cable they're quoting. I've also reuploaded the images to Imgur here, because it looks like that HK01 story has already mysteriously disappeared down the memory hole a couple of times.

Follow up question. Do we have any accounts of protesters killing police and soldiers BEFORE the crackdown?

When I was in China one of my friends, who had no reason to lie and who was a preteen during the crackdown, told me in a private conversation he had seen dead police officers strung up on lampposts several days before the military moved in.

Being a Canadian I had only heard the "government bad and evil vs protesters good" narrative.