It is impossible to prove a negative, as you know very well.
I have access to online law journals, which require a subscription, and a search produced no results.
In some American states it is illegal - mainly as a reaction to the KKK.
Also, the BNP and other national parties have lawyers amongst their members, who would have been searching for such a law for a long time.
The police too - in reaction to the prayer meetings held on some streets where the followers hide their faces.
And I'm sure the red-top papers would have found such a law to further there diatribe against Muslims.
So, I am 99% confident no such law exists in the UK today.