You've created a false equivalence between two things: the right to free speech and the right to a platform for that speech. You have the former, it's a negative right like all natural rights. The later is not and you don't have it.
If I own a soapbox and megaphone for people to use while they express views to a crowd I am not 'censoring' anyone by not allowing them to use my soapbox or megaphone. If you believe I am, may I please have your address so I can come round and recite the complete poems of John Milton everyday? You wouldn't want to support censorship, right?
Cardiff University is not a publicly owned institution, it has no obligation, legal or moral, to provide a platform and it has no place using the lofty values of free speech to defend itself from judgement when offering a platform to a bigot.