Hmmm... premeditation does not preclude diminished responsibility: a schizophrenic might make elaborate preparations for days to murder a complete stranger, but it would still be manslaughter not murder as he would not be fully responsible for his actions. "Premeditation" is not a test for murder these days and has not been for decades. Even if it was, there would still be a partial defence of diminished responsibility available in the appropriate circumstances.
It sounds like this woman had no intention of "getting away with" what she had done - she made no attempt to hide it and made a full immediate confession. To my mind, she must either have been a bit mad (diminished rep) and/or there must be a lot more to the domestic abuse that didn't come out in the trial...
Even if she was in her right mind and so fully responsible for murdering her DH, it seems like a waste of money (as well as another life) to be keeping her in top security jail in her seventies - do they have the facilities to care for someone who is ageing? - and the only danger she poses to anyone is to herself..... Surely 10 years would have been enough, until she is 67? Baby P's sadistic killer only got 10 years, but that was before the new statutory minimum sentences were brought in.
Hmmm...lack of remorse - covering up the body, leaving a note saying "I love you" and putting a cushion under his head to "make him more comfortable" is not exactly a sign that there is remorse, but nor is it a sign that she was jolly glad about what she did. It's the illogical, bit mad action of someone in complete shock.