Under U.S. contract law (and I would assume UK as well), what the groom and bride are doing is contracting for the services of the photographer to include both taking the pictures and the printing of the pictures, not for the intellectual property rights (copyright) of the images. In many cases, the taking of the pictures is done gratis or for a small fee to pay strictly for the time s/he is there. If you don't like the images, then you don't buy many prints. But if you liked the images, then you buy more prints. That's the incentive for the photographer to do a good job.

You could (theoretically) take the proof folder, scan the images, send them to an online or brick & mortar print service, but in nearly all cases the print service is going to reject the order on the basis that you do not have permission from the photographer to make copies of the images. Whether it be professional courtesy or a good case of CYA, they're not going to print your prints.

For a sizable fee you may buy the "negatives" to the images so you can print all you want. Back in the day it was the no kidding negatives but these days it is typically a full-sized TIFF image. Even with the TIFF's, the print service is going to want to see notarized documentation releasing the copyright.

Almost without exception professional photographers will work under this premise unless prior arrangement is made. Prior arrangement from a professional photographer will come with a big price tag. If it doesn't, worry about the quality that you're getting.