Before I start, I must say that I have a 7 month old baby who is up every hour at night, so please bear with me if I can't reply to your comments. I'm a zombie at present 
I speak on behalf of Muslim women in the UK who wear niqab. Many of my friends and family members wear niqab and I myself wear it too on occasion. I have no idea how others around the world who wear niqab feel.
Muslim women in the UK wear niqab because they:
Want to please their Lord.
It really is as simple as that. Yes, there may be one or two people here and there who may be wearing it for other reasons (like wanting to fit in or please their husband), but the truth is 99% of women wear it because they want to enhance themselves spiritually and believe it will bring them closer to God.
It is extremely extremely difficult to wear niqab. You are constantly ridiculed, feel under threat and people not only look down upon you but give you abuse too. You must trust me when I say that these women have not made this decision lightly to wear niqab. There is absolutely no way that a woman would put herself under such pressure for someone else (like a husband for example), they would only do it for God.
It is extremely sad that women are judged because of their clothing (whether because they cover too much or not enough). I don't judge women who wear just a bikini for example, I would hope that people wouldn't judge me because I choose to cover.
However, in all honesty, I must say that generally I have seen that my fellow Brits are open minded and let women dress the way they want to.
Please, next time you see someone wearing a niqab, stop them and ask them why they wear niqab. Trust me, it'll make their day 
On an ending note, why do I feel like I should have name changed for this? Will people treat me differently now that they know I am a Muslim and wear niqab?
Right better dash - baby calls!