Beginning with the UKUSA Agreement of 1947, the five participating countries worked closely together in the area of signals intelligence, or SIGINT. In the beginning, this included mainly exchange of information gained by one of agency and needed by another agency. This exchange had to be done manually.
In the 1970s, the first computer systems to intercept and eavesdrop messages and phone calls were created and used. But this was apparently still done independently in each country.
A newspaper article from August 7, 1980 that I couldn't find wrote about a U.S. satellite and communications base at Menwith Hill. near Harrogate in Yorkshire. According to this report, "it is run undercover by the NSA and taps into all Britain's main national and international communications networks" (reference found in ).
In , I found a remark, that in the early '80s, the NSA was developing a system called Platform, which would integrate at least 52 separate SIGINT agency computer systems into one central network.
Another article from August 12, 1988 () reported details about a global surveillance system called "Project 415". This system seems to be closely related to the Echelon which was about to be implemented. The article states that in Britain, the GCHQ had 15,000 staff and a budget of 500 million pounds.
When exactly Echelon was created as it exists today is hard to tell. Probably, the system Platform eventually became Echelon. The earliest definite date I found was about New Zealand joining Echelon in 1989 ().