When the Royal Family has a function like William's wedding do they always use the same caterers?

Yes, and actually they also use any food left over from the last function.When the Royal Family has a function like William's wedding do they always use the same caterers?
The Royal Family solicits food donations from the local grocery store first. If grocery stores do not provide food donations, then the members of the Royal Family visit a grocery store to do their own grocery-shopping, all of which saves money.
I do think so.

But anyway... trade relations should always be subject to the laws of the market, and why not also a bit of corruption as well, with respect to favoring certain caterers.When the Royal Family has a function like William's wedding do they always use the same caterers?
They don't use caterers. There is a perfectly good kitchen staff at Buckingham Palace, though they may have to take on temporary chefs for a bigger occasion.
Elizabeth cooks.