I was at Longleat last weekend and we were handed a notice warning of thefts of “Custom Built” and “Boutique” bikes, whatever the latter is.
They also recommended that you lock your bikes inside your accommodation at night.
I only took my “4th” bike, so just opted to leave outside with a heavy duty cable and D lock.
We didn’t have any issues, but there was quite a bit of construction/maintenance going on, so although it is mostly vehicle free, there were quite a few vans going on and off site. Therefore it would be quite easy for someone to throw a couple of bikes in the back of a van and get offsite with ease.
I would agree with previous posts, unless you are going to ride in the surrounding woods, (which by all accounts has some half decent trails), a hire bike is more than adequate and takes away the stress of worrying about your own bike.
I was planning to venture offsite, but in the end just didnt get time due to all the other stuff available on site.