A petition to Parliament only requires 10000 signatures. The petition mentioned above hopes to raise 500,000.
The issues having them concerned go beyond the simple arrest and imprisonment of Robinson. It is about freedom of the press and due process. Banning reporting of his arrest and quelling any discussion in the MSM reeks of a hidden agenda.
The following youtube clip alludes to this idea and takes the discussion far wider than Robinson.

https://www.youtube.com/watch?v=3qPy051z_PE
It is a speech by Farage to the mainstream media. I recommend you listen and form your own opinions.