Well, it happens!
.... But not as often as you might think. Britain is notoriously wet, but the reality is that on any given day there will be non raining windows of opportunity. This might not be convenient and might give you and your photographer a bit of a headache, but even on a wet day it is often possible to grab half an hour when it's dry.
Even so, it's always advisable to have a contingency plan. I have done many weddings where, pretty as the outside was, the indoor "Rainy Day option" was stunning. It's not always the end of the world!
Whatever the weather, try and chill out. If it rains have an umbrella at the ready, maybe even some dazzling wellies. It's what happened on your wedding day, embrace it and enjoy it!