When was the first motorway built in England?

The first motorway opened in Britain was the M6 (M=Motorway). It was known as the Preston ByPAss. It was opened in 1958.
When it was planned, the Preston ByPAss was seen as the first piece of a large network. It was deemed a success and led to the construction of a second motorway, the M1, opened in 1959.