The law would start by looking at the T&Cs in the contract and we can only guess what they are, you'd need to read them. Assuming the package is through a UK tour operator the likelyhood is that there will be a cut off date. Cancel before that and they may get something back less a penalty charge which will increase as time goes by, so they need to do it without delay. Cancel after the cut off and they may even be liable for the full amount and have to pay more. Certain special wedding components may have seperate conditions, none of which will work in their favour!

The contract will name of one of them as lead person, only that person can inform the operator and it needs to be done in writing - letter, not email and make sure the booking confirmation number is clearly shown. If the booking was made via an agent the letter should go that way (and any refund will probably come back that way). If the letter needs to be posted it should be sent recorded delivery.

It's normal for tour operators to take 28 days to answer a letter so they can expect a wait.