This is impossible to answer, really, because it depends on what interests you. Personally, I would not like the Merlin pass, because I would have little interest in most of the attractions to which it gives access, but its tastes are almost certainly different from mine. I have no idea if you would personally enjoy Madame Tussauds or if you would spend that time better in one of the world-class museums in London (British Museum, V&A, Tate, etc.), touring the Tower of London, perhaps an exclusive tour of Buckingham Palace (for which there are only a few tickets left in December; yes, it costs £ 85, but you get a small group guided tour of Buckingham Palace), or maybe a West End show. Or maybe you would like the London Transport Museum or Churchill War Rooms or Bletchley Park or a tour of Parliament or a shopping trip or tea or … my point is that there is a lot to do in London, and you will have to choose what you personally find interesting.
I would get a guide from London and take the time to investigate the things in London that interest you most. Combined cards like these are generally only worth it if you really want to visit all or most of the things on the list. Wax museums and aquariums that you can see in many places, while there is only one Tower of London. Many of London's museums are free to enter, so a pass does not help you there, and other attractions allow you to buy tickets in advance directly online. Therefore, I would be inclined to buy a card, but it is worth adding the costs once I know where you want to go to see if it will save you money.
As noted in the comments, many attractions will be closed during the holidays, which will influence your calculation.
If you want to go to the Warner Bros. Studio Tour for your Harry Potter attraction, your site provides information on how to get there by train, or you can scroll down to "Bus Transfers" if you wish to purchase a combined ticket with a bus from London. , as Bernhard Döbler points out, prior reservation is required, and there may be no tickets left for your dates.