|Location||National Trust Blickling Hall Norfolk, UK|
|Extra info||photographer thinks: R. Pink Pearl|
Looks like Loder's White. While it is not listed as a Group, I know I have seen at least two forms of it. One gets more pink in the center with age. Is this Halopeanum (aka White Pearl)? Looks like Loder's White Yest it is the form of 'Loder\'s White' that gets more pink as it ages. No question about it.
See high-resolution picture