I've only read one KJ Charles before, A Charm of Magpies, which I really liked, but I didn't feel compelled to read the rest in that series. Now that I've read Think of England, I am firmly a fan!
First, I've always loved m/f regency romances, but it's hard to find a good m/m romance in set in anything but contemporary England for obvious reasons. This book handles that brilliantly. It is absolutely true to that era and uses the difficulties as part of the obstacles/plotline. The attitudes and lingo all felt very accurate. It still has that sort of 'cozy' Agatha Christie feel I love while also being a hot m/m romance. Bravo! That's a tough nut to crack (no pun intended).
Second, I adored the characters. Daniel is gorgeous with his wicked cruel tongue and sharp wit. I loved how he got them out of a jam several times just using his brain (even against weapons). I loved the foppish air he affected or dropped just as easily. And his 'Viking', Archie, is as stolid and steady and English as they come. I loved the slight GFY twist to this and loved Archie's bullheaded plodding forward regardless.
Third, the writing is just damn good. There's not an excess of flowery meandering but every line is well-written. The dialogue especially was just awesome.
Fourth, I also liked the secondary characters, most specifically the two ladies who helped save the day. Nice to see non-fainting femmes.
Fifth, I'm not much of a fan of continuing series with established couples, but KJ did a brilliant job of giving me some hot scenes with Daniel and Archie while holding enough of their relationship back that I definitely want to read the next book.
If you haven't tried KJ you definitely should. She's one of the best m/m writers out there IMHO.