Why is an Abaya more “Islamic” (whatever that means) than a full-sleeved Shalvar Qameez? Because what Arabs wear is automatically more “Islamic” than what we wear?
To the OP: It sounds like you’re already married from a religious perspective, and all that’s left are British legal formalities. I don’t quite understand what you’re trying to do…you want to know the proper Islamic manner for observing a legal formality that didn’t even exist in the Prophet’s era? That really doesn’t make much sense.