I'm really really confused.
Why do you need to rent a car?
Before I chop off your head, perhaps there is a critical reason for that?
If you have never driven to Windsor and driven around it for 90 minutes looking for a suitable parking like I have, you might still think it is a good idea. I'm here to say it often isn't. It is a small town which gets loads and loads of visitors every day. It is absolutely trivial to get to by train, about an hour from Waterloo, or about half that from Paddington and the train from Paddington stops just opposite the Castle. Paddington is 5 simple stops on the clockwise Circle Line from South Ken.
A very simple and fast train ride from either Euston Station or Harrow and Wealdstone to Watford Junction puts you just across a small drive to bus stop number 4 which has a very special Bus with special decoration which takes you straight to the Studio Tour.
South Ken on the Piccadilly Line to Green Park to the Victoria Line to Euston and upstairs to the London Midland trains to Watford Junction at xx:49, xx:54, xx:24, or xx:34 most hours during the day which takes between 15 and 20 minutes for the trip.
Or South Ken on Piccadilly Line to Kings Cross and back one station to Euston on the outbound Northern Line Bank Branch, and above. Or Piccadilly Line to Leicester Square and northbound on the Charing Cross Branch of the Northern Line.
Or South Ken on the anti-clockwise Circle Line or District Line to Embankment to the northbound Charing Cross Branch of the Northern Line.
all of those are your choice.... depending on loadings, any delays, or your preference.
I hope you aren't trying to do both of these on one day - it would be absolutely exhausting.
Just hiring a car to go to the Harry Potter would be a colossal waste of time and money. Public transit would be so much easier and faster.