Unfortunately, I haven't been there myself - I've got completely different museums on my priority list, more focused on nature and health stuff. But I've heard about Trent Park from colleagues who are into history: it's actually a really cool place if you're interested in Second World War history, especially intelligence and counterintelligence. They show how the British set up eavesdropping on German officers in this mansion - it was a pretty clever operation, with guys literally living in the house and intercepting conversations.
As for the exhibits - the museum recreates the interiors from that period, and there are genuine documents and artifacts. You can see the officers' bedrooms, listening rooms, all kinds of ciphers and equipment. Plus the park grounds themselves are beautiful if the weather's nice. You'll need to check the official museum website for tickets - they always have up-to-date info on dates, prices, and whether you can book a guided tour if you'd prefer that.