Muslim Woman to uncover her neck in front of Muslim women and non-Muslim women
In the name of Allah, the most Beneficent, the most Merciful.
The awra of a woman in front of fellow Muslim women is from the navel up to and including the knees. (Hidayah p.461 v.4)
The awra of a Muslim woman in front of a non Muslim woman is like that of a non Muslim man, i.e. the whole body besides the hands, face and the feet. (Raddul Muhtar p.534 v.9) However, scholars have said if this is not possible to do then one can expose some parts of their body.
In conclusion to your question as there will be a mixture of Muslim and non-Muslim women at your wedding it is advisable and preferable to cover your whole body besides the face, hands and feet. If a bit of your neck is exposed then it will be considered permissible according to the second view.
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham