Your wedding is probably the most important day of your life and you want your photographs to reflect that. For that reason, my advice is to hire a professional photographer to capture your day as it unfolds.
A professional photographer will have had years of experience and not necessarily only in Wedding photography. They will have received training in the art of photography and spent years honing their skills and developing (pun intended) their own unique style. More importantly, they'll be able to work quickly, often under pressure and have people skills. Try prising guests from the bar for that all important group shot and you'll see what I mean.
You might think you'll save a few hundred pounds by asking a friend or a friend of a friend who just happens to own a camera. Will they have the necessary skills to capture your day to the same level of quality as a professional and will they be able to work quickly and under pressure. Wedding photography can be very stressful and if not approached correctly then the results could be disasterous as you only get one chance to get it right. Just like life itself, there is no rewind button.
"If you think it's expensive to hire a professional to do the job, wait until you hire an ameteur." (Red Adair - Professional Oil Well Firefighter.)