Snapfish sounds like it would serve all your needs:
They allow you to share an album with people, and then the people you've shared the album with can order prints and enlargements from that album:
Relevant FAQ entries:
What does it mean to share a photo or album with a link?
With the address (URL) link you can choose when and where you would like to share your photo or album. It's a direct link to a web page with your photo or album. Copy and paste the URL into an email from your own email provider, in an instant messenger chat, on a blog or social network, or wherever you choose.
Once I've shared a photo or album(s), what can my friends do with it?
By signing in to Snapfish using an email address and a password, your friends can see the album anytime, plus order reprints and enlargements of any pictures they choose.
Can people I share with order reprints? Who pays for them?
Yes, anyone you share a photo or album(s) with can order reprints on their own, at no cost to you! When they order reprints, they will submit their own billing information. You will never be charged for an order that someone else places.
I've ordered prints and photo books from Snapfish before (but in Australia) and I've been satisfied with the quality of their individual prints. They should be good enough for your guests. Shipping was also extremely fast - as I said, I'm Australian, but they also operations located within the UK so you can get local shipping.