There's something called the Chunnel --you can take a train from London to Paris, and from there to anywhere else in Europe. She doesn't have to fly, though she could, no doubt. She just does what she wants.
She is 93. I believe she really cannot fly and traveling by train for so many hours, it would be exhausted and even dangerous for her health.
She could have made a public statement though. That's sure.
I've known a few people who have lived well into their 90's, and one into her early 90's now. They do not travel much, if at all; tend to fall into certain routines, and smaller circumstances. Hence the DoE preferring Woods Farm at Sandringham? Anyway, while I don't fault her for not going to the funeral, yes, a public statement of condolence might have been made. Did Ed a Sophie go to the funeral? Perhaps she thought that was enough.