The outfit is lovely but only buy it if you're going wear it again. People of the Muslim faith living in the UK see other people's skin every day and don't fall down dead. The faith is about being modest yourself not dictating the clothing of people of other faiths or none.
You colleague has invited you, a non Muslim to her wedding and sees what you wear everyday. I am guessing she has also invited her other non Muslim friends to her wedding. I pretty sure she and her family and friends are well used to working and socialising with non Muslim people. This being the case, I am 99% certain that if your outfit isn't over the top on the skin showing front - which after all is what is most people would think appropiate for any church wedding, it would be fine for this wedding.
That being the case, if you have already have something suitable, wear it. If you want something new buy it.