By now, Christopher Steele has become a household name. He was the author of the controversial dossier alleging collusion between Russia and Trump.
Steele is also a former Secret Intelligence Service (MI6) agent. He retains close ties with former MI6 head, Sir Richard Dearlove. Steele, and his business partner, Christopher Burrows, consulted with Dearlove while they were in the process of producing additional documents for the dossier.
A good point to start is in late 2015. Britain’s Government Communications Headquarters (GCHQ) was involved in collecting information regarding then-candidate Trump and transmitting it to the United States.
As Luke Harding of The Guardian reported, “The precise nature of these exchanges has not been made public, but according to sources in the U.S. and the UK, they formed a suspicious pattern. They continued through the first half of 2016. The intelligence was handed to the U.S. as part of a routine sharing of information.”
https://www.theepochtimes.com/the-uk-and-australia-have-reason-to-be-concerned-about-declassification_2671461.html