Following the success of the Atom, Acorn started to look at developing a more powerful successor, the Proton. This was to include a 2MHz 6502 processor and more expansion options including the ability to add a second processor via a Tube interface. A prototype was built, which is now on display at the Centre for Computing History in Cambridge.
It's development changed direction when the BBC computer literacy project started looking for a BBC computer. Acorn was one of several companies approached to see if they could meet the requirement. Fortunately for Acorn the Proton was sufficiently advanced and with some rapid development the Proton design was able to be adapted to become a candidate for the BBC's project, it exceeded the requirement and of course became the highly successful BBC Computer.