I have found a Kew Gardens 50p in the past. I believe I would have found more, but obviously everyone was looking for them and removing them.
No one - other than myself - has been actively looking for these 2006 ten pences. I have four. Three I've found, one AU/possibly UNC from eBay earlier this year. I know Tony Clayton has at least obe, possibly two of these. I did hear of someone else who had one. So that's what seven known coins for certain. I suspect perhaps only one die was used for these and I just happened to acquire two of them close together in 2006/7. I haven't seen one since until this morning, and I have been looking!
There's nothing rare about the Kew Gardens 50p. I could find countless on eBay or elsewhere and buy a mint state one tomorrow, for a price obviously, but they're are readily available, expensive doesn't equal rare!
I hope you can update your coin book with these varieties!