I simply cannot fathom why someone would wish to travel to the other side of the globe to spend three days looking around Harry Potter world.
Absolute waste.
Far too packed.
I’d suggest maybe a day looking around Hampton Court? If you can time it with Evensong in the Kings Chapel, they’ll thank you for it.
Nothing else around there though except hotels. Molesey is a dump. So head off into London after.
Scoot up to Winchester. Fantastic day...