Which people should I ask ? The author himself, UK book publisher, US book publisher or Universal movie studios ? Also, I don't know what deal the author made with Universal...if he sold his rights to them or not...

You should probably start by contacting the author's agent. The specifics will depend on what you want to license and what licenses have already been granted by the author to other parties, and what rights those licenses give all parties involved. You'll probably want a lawyer involved as well.

Practically speaking there's a very slim chance this will work out favorably for you.

Now, the first two of them have a movie made by Universal. Which people should I ask ? The author himself, UK book publisher, US book publisher or Universal movie studios ?

You should do the research to find out who owns the rights and approach them.

Also, I don't know what deal the author made with Universal...if he sold his rights to them or not...

You need to find this out too.

It also depends on the how the books have been licensed. It could be that somebody already has exclusive rights to make a game based on the books, or it could be that the rights are non exclusive so that multiple studios could licence them.
Also just having the licence doesn't mean you can go and make the game you want. Licences usually come up with restrictions about how you can use them. Also since the book has been made into a film if you got a licence to make a game based on the book then you would need to be careful that your game doesn't resemble the film otherwise you would be in trouble with Universal.

Practically speaking there's a very slim chance this will work out favorably for you.

I'd tend to agree with you but, lets not jump to conclusions. The OP hasn't detailed which book he is talking about. I worked on a game at a 5 man studio that had got hold of the Tarzen licence and it was very easy and very cheap.

If he wanted to licence "The Hunger Games" then obviously its not going to happen but, there are other IPs out there that can be licenced fairly easily by an individual who approaches the right person.

Practically speaking there's a very slim chance this will work out favorably for you.

I'd tend to agree with you but, lets not jump to conclusions. The OP hasn't detailed which book he is talking about. I worked on a game at a 5 man studio that had got hold of the Tarzen licence and it was very easy and very cheap.
If he wanted to licence "The Hunger Games" then obviously its not going to happen but, there are other IPs out there that can be licenced fairly easily by an individual who approaches the right person.

I'm talking about Wardstone Chronicles series by Joseph Delaney, and the movie is The Seventh Son. Since Joseph didn't worked with the screenwriters and each one of them wanted to change something, the script for the movie was almost completely different than the books. Also, the movie didn't go very well, so there might not be a great interest from the movie company in sequels.

Funnily enough I watched this film last weekend. I am also originally from the area in England that "The County" is based on.

Your information is incorrect. Seventh Son did receive bad reviews from critics but also some favorible. It is also considered a box office success taking the number 4 spot at the box office at its US release and also having made a profit. Also a small amount of Googling shows that the production company Legendary Pictures granted various Chinese games studios with rights to make games based on the movie (these probably were / will be mobile games).
The fact that the script differs from the book is also irrelevant and only worth mentioning for the annoying book whiners on Facebook who moan at the slightest change or plot point that differs in movie adaptations of their particular story.

None of this has any baring of how likely you would be to get the licence. At the end of the day since there is a recent film based on the books that has also just hit streaming services so is still making money and at least one studio has made or is making games based on the film then the author or authors agent is more than likely going to be demanding a premium.

If you want to make a game you will need a fat stack of cash to pay for the licence. If you plan on making a fan game or putting anything on Youtube without a licence then prepare to Lawyer up and get sued and loose.