ATV wasn't exactly a channel as such, it was more of a production company based in the Midlands (although its studios were miles away in London) that aired it's programs on the UKs ITV channel. It disappeared in 1982 when Central Independant Television took over and moved the studios to Birmingham where the company was based. ITC was a subsidiary of ATV that distributed their programs out of the country, mainly to the USA.
Lew Grade was the guy that gave Jim Henson the chance to make The Muppet Show at his ATV studios in London because USA networks weren't sure it would be a success. ATV then sold rights to other networks in the USA to show the series.
Side note: Lew Grade's nephew, Michael Grade used to be chairman and controller of the BBC in the UK from 1984 until he resigned to become Executive Chairman for ITV in the UK this year.