Groupon Offer ” XYZ Wedding Photography for £400″, Groupon commission 40% £160+vat= £192 XYZ photography get £208, of which XYZ have to pay a photographer to shoot the wedding and XYZ may also have to process/edit and deliver the end product AND make a profit. XYZ can book more than one wedding per day, but the problem arises when all the regular photographers are already booked.
This model idea was available many years ago under the name National Weddings and later Kodak Weddings, where you met your personal photographer on your actual wedding day. There were quite a few unhappy brides. I believe both National Weddings and Kodak Weddings are no longer trading.