As an American who spends a frightful amount of time in the UK, I found this book to be hilarious, but then I am familiar with a lot of how the UK works. For this series, it probably helps although it isn't absolutely essential. If you watch a lot of Mystery or read a lot, then you'll get by fine.
This book is dry British humor. It holds no sacred cows in fact she targets and sends up sacred cows in her books. She also has a cat character, Plutarch, who is one of the funnier cats in writing without that nauseating cloyness that many authors give pets. Then again, that's probably part of the humor.
If you like dry humor, if you appreciate poking fun at most everything, if you like intelligent slapstick (do you like Mr. Bean?) then you will probably appreciate the humor here. It also helps if you understand and appreciate irony. Otherwise, much of this may either bewilder or bore you. If you're a bit prudish this probably isn't your cup of tea either.
One other thing - the mystery here is just a vehicle for a hilarious story. If you want mysteries to be the key, this will probably irritate you too. I wasn't particularly worried about the solution as I was enjoying the caper far more, but I knew what I was in for.