I think that whilst people don't think of Twitter as a medium of 'the press', it's fair to look at it that way. We have mostly free speech and free press in this country, but there are exceptions against inciting racial hatred. Wikipedia says " The Criminal Justice and Public Order Act 1994 made publication of material that incited racial hatred an arrestable offence.".
So, since he was publishing these comments on his public Twitter page, I think that it would set a dangerous precedent not to apply the law to such conditions.