My understanding is this:
It is an offence to drive on the footpath. However, from case law, a judge ruled that a police officer has to witness the driving part to make a prosecution, not up to the driver to prove he didn't. Could theoretically lift it there!
In some municipal areas there is a local bye-law prohibiting parking on footpaths. I believe London is a blanket ban, unless specifically allowed, as the sign above shows. Other areas may have rules specifically for some streets.
As Ziggy says, many areas give no alternative, so everyone does.
Consider the benefits. Have you created a bigger space, that allows traffic to pass faster, so increasing your risk. If you park on the road, is the space left only just big enough to pass opposing traffic, so your risk is lower when on the footpath.
If in doubt, don't.