The places you have mentioned are in three of the 'home counties': Herts, Hants, and Surrey, and I have tooled around two of the venues (never been to the Harry Potter film sets). It's debatable whether it makes sense to rent a car to visit popular tourist venues in the home counties, but that's a topic relying heavily upon your experience in South East England combined with your agenda. The home counties are thought to be safer than say Brixton or Wood Green, but it is not prudent to assume this.
Some of the hotels in those locations have a car park located either under the hotel or in a flood lit area, but even in the best of conditions you wouldn't want to leave your stuff in a car overnight. Put simply, it's poor form.
The rule goes for stuff that's visible or stored in the boot. Nobody will sympathise if anything goes missing or if the car windows are damaged; you can expect the police to sigh and roll their eyes. If you want to keep your stuff safe overnight, carry it into the hotel.
In the day time at those venues, it's generally ok to leave your stuff visible if you are in the venue's car park (but not the overflow car park), but even then people like to cover it with a car blanket throw stowed in the boot for that purpose. Look around at what other people are doing, the global rule "monkey see, monkey do" applies.