Thank you for your answers.
Not only would my question apply to veiled women (or men in disguise - you never know), it would also apply to men or women wearing balaclavas, or scarves wrapped around their faces, covering their mouths & noses, 'specially in the cold winter weather!
In the past, both men & women wearing protective hooded jackets have knocked on my door in such bad weather & after carefully reading their identity tags, have been happy to stay by the door, whilst they read the meter/s. I expect it's because I could see their faces on opening the door!
I wouldn't let anyone in my house dressed in one of these either:
http://en.wikipedia.org/wiki/Balaclava - I would ask them to remove it, the same as I would a veiled person.
P.S. I met some really lovely Muslim nurses at the hospital yesterday, so my question has nothing to with Muslims in general - it has however, everthing to do with not knowing who is under full veils - that's all.