Not raising the threshold at least in line with inflation is raising taxes every year on those that can least afford it as well as dragging the most vulnerable into paying tax. My mum is also on the minimum pension and when my dad died and that halved the household income, his 2 tiny private pensions worth about £2000 a year, last years pension rise took her over the threshold.
Not sure when the threshold was last raised but it was a fair few years ago now.
It's a pisstake. The lower end £12,570 should never have been frozen.