I know this is an old thread, but several Angela Thirkell books appear to be available (or available for pre-order) at Kobo in the UK:
http://store.kobobooks.com/en-gb/Sea...ngela+thirkell
Christmas at High Rising is also available in the US Kobo store. Even more titles show up for pre-order in the Amazon UK Kindle store.
I haven't read any of her books yet, but plan to put them on my TBR list.