Because the best outcome in all of this is that you take the cash equivalent of what people had tried to spend at the department store, find your dinner set somewhere else, and buy similar pieces yourself. If you can't get actual cash from the department store, but you would have spent an amount equivalent to the gift vouchers there eventually, then you can just use your own money and all is well. But if you wouldn't otherwise have spent that amount of money at the department store, then gift vouchers are rather useless to you and you might consider pressing the store for an alternative solution to this problem.
I'm also curious how many people tried to purchase things off your registry, and how close to you or your fiance they are. And also if the department store would be willing to issue refunds to the original purchasers. Because if a close friend or relative came to me and explained a similar situation, but told me that the store would issue me a refund and they were registering for the same dinner set at a different store, I'd absolutely take that refund and go buy something from the dinner set at the other store. My goal as the gift giver would be to give the bride/groom what they actually wanted, and as long as it wouldn't require lots of convoluted steps for me to get my money back and buy the thing from the other store, I'd be more than happy to do that.
I wouldn't personally be comfortable asking anyone but a close friend or relative to take a refund and use it somewhere else, and I think how I'd react to that as a gift giver would sort of depend on the circumstances. But if there aren't many people who have tried to buy off the original registry, and they're all close enough to you that you talk to them on a regular basis, you might ask the department store if they'll issue refunds to the purchasers if they refuse to issue an actual refund (in money, not gift vouchers) to you.
This part of Dindrane's post got me thinking. I'm from the US, so I'm not sure what is common practice in the UK, but the store practices seem shady to me. In the case of ordered merch not being available, shouldn't the default scenario be to refund the person who spent money to order the merch?
In any case, i like the suggestions of the grapevine, but I'd maybe escalate this issue to department store corporate, if possible.
The reason the store isn't just refunding the purchasers, which I agree it should, is that it has the money they spent and it wants to keep the money they spent. Simple as that.
Refunding the original purchasers means they lose the money. Giving cash instead of a store voucher to the OP means they lose the money.
If the UK store is anything like the US store where I used to work, salespeople are tracking on how much they sell. The bride consultant is tracked on how many registries are opened, the dollar/pound value of the things on the registry, etc.
Losing the sale by refunding the money and/or losing the registry by having the OP close it hurts their sales figures, so they try to make that not happen.
I do not know if refunds are a possibility at this store, but the OP should pressure them a bit to see if it can be done. They aren't going to volunteer that option, but they may admit it's a possibility if asked directly.
And the staff in the store probably didn't know that the dinnerware was about to be discontinued. I worked in the home department of a large US department store chain, and while someone in the corporate offices would know what was being discontinued, we did not. We learned when a "closeout" sale sign went up, or we went to order something for a customer and found that we couldn't anymore.