Like most say it depends on the circumstances and the relationship, but £500.00 no way.
My opinion is if you have to ask guests to pay you really cant afford the type of wedding your aiming at.
Better to come down off cloud nine and cater within your means, family and friends will understand how expensive weddings are, and if they grumble you can do without them, they are not worth a second thought.
Okay to ask for money instead of a present if your already living together.
I think Carolyns son and future DIL have a wonderful idea with the picnic and hope everyone has a great time especially the Bridal Party. Gill x