Sorry, JoBrodie, anonymous reporting is not fine. There are, obviously, plenty of circumstances in which anonymity can be necessary -- if you are reporting a neighbour you know is likely to be violent towards you.(Even there, what are you going to do when you are subsequently asked to testify in court?) OP just wants to think through what is the right thing to do. And that should include her asking herself why, if she's so sure, she's not willing to stand up and be counted. And indeed, whether she is actually right in her opposition to this party.
Here, as raspberry pointed out with a different intention, there are no bombs falling. This is not a war, and the government did not suspend habeas corpus, meaning technically the Coronavirus Act may be unconstitutional. (All the convictions attempted in the first national lockdown were ultimately thrown out by the courts, remember?)
More to the point, there is a valid argument that it is unfair and unreasonable.
I refer you to the website of the Office of National Statistics, where you will learn, among other points, that in the full year to Nov 23, exactly 286 people under the age of 40 died with covid in the UK. Also that, in fact, the average age of people dying with covid is higher than average life expectancy. You will have to pry much deeper to establish that the majority of infections in our "out of control" situation are taking place in group living situations: hospitals, care homes, and prisons. (That's one of the reasons Kent is doing badly right now -- prisons...) Arguably there are better ways of protecting those people, to whom we certainly owe protection, than destroying everyone else's lives.
For you may go to the Office of Budget Responsibility, which will inform you that it expects GDP to drop by £239 billion this year, extra government spending to hit £400 billion, and unemployment to double to 2.6 million people -- and guess which generation will be paying the price for those figures their entire working lives.
Putting this in more concrete terms: it is entirely possible that OP's daughter cares more about seeing her friends for Christmas than seeing her 84-year-old granny. OP might want to discuss the options with her before going nuclear.
As a practical aside, if they're 21, chances are they're all university students -- in which case they will all have been tested once or twice very recently before leaving for home.
But, without being unduly dramatic, I'm telling you: as a free citizen, I'm obeying the rules because I choose to, but I would sooner we all got covid than live in a country where people felt "moral" reporting their "friends" anonymously to the police. This is not China or Iran or Syria.