It's really impossible, it depends on what interests you. Personally, I would not want the Merlin Pass, because I would have little interest in most of the attractions to which it gives access, but your tastes are almost certainly different from mine. I do not know if you would personally like Madame Tussauds or if you would spend more time going to one of London's most prestigious museums (British Museum, V & A, Tate, etc.), to visit the tower of the Tower of London. London, perhaps an exclusive visit to Buckingham Palace (for which there is only a handful of tickets in December – yes, that's £ 85, but you get a small group tour of the palace Buckingham), or perhaps a West End show. Or maybe you prefer the London Transport Museum or the Churchill War Rooms or Bletchley Park or a visit to Parliament, shopping, tea or … I think there is a lot to do in London and you'll have choose what you find personally interesting.
I would like to get a guide from London and take the time for you to research the things in London that interest you the most. Combo cards like these are usually only worthwhile if you really want to visit all or most of the items in the list. The wax museums and aquariums that you can see in many places, while there is only one tower of London. Many London museums are free, so a pass does not help you, and other attractions allow you to buy tickets in advance directly online. So I would be against buying a card, but it's good to add up the costs once you know where you want to go to see if it will save you money.
As stated in the comments, many attractions will be closed on holidays, which will affect your calculation.
If you want to go to Warner Bros. Studio Tour for their Harry Potter attraction, their website will explain how to get there by train. You can also scroll up to "Bus Transfers" if you want to buy a ticket combined with a one-way ticket. London bus. However, as Bernhard Döbler notes, it is necessary to book in advance. There can not be any tickets for your dates.