Roxy is a cuddlesome doggie. After three years in a last chance rescue centre, I think anyone would feel a little insecure. One of the things that she enjoys most is having her belly rubbed. Of course, for a female dog, this means contact with the nipples (as has been previously documented). Normally, this is how Roxy shows she is after a belly rub...
In less energetic moments, Roxy likes to lean in for a cuddle while sat upright. Once you begin to stroke her, Roxy will then lean a little more, so you can then reach her belly. Cuddle AND a belly rub? That's a happy dog right there. This isn't a belly rub shot, but it shows the quietly happy pose Roxy can strike. The warm sun probably had something to do with it.
As Julie rightly pointed out, Roxy can look positively noble - even regal - when she is serenely happy. In fact, Roxy apparently looked so regal on one occasion she was receiving a cuddle/rub, that Julie was moved to draw a comparison to our own, dear monarch.
You'd think, after all this time, I would be more careful about having a mouthful of tea when Julie says things like this... Mind you, I don't imagine the Queen was too overjoyed either.