Not even a judge? We don't have a system of career judges, unlike, say, France, where they start as very junior investigating magistrates and work their way up. All our judges have been counsel or solicitors. The old Judicial Committee of the House of Lords (now The Supreme Court) occasionally had a member who had previously been an academic, so, in itself,even that is not an objection.
Yes, the man who represented the hate preacher might have been just the man to get him ejected from the country!. 'Poacher turned gamekeeper' comes to mind. A lot of counsel both prosecute and defend ; the prosecuting only practice is a rarity. I've a feeling, though, that professional etiquette would stop him acting against his own, erstwhile, lay client in what is effectively the same cause!