What about the more remote stations?
I think it is sometimes easier to adapt remote stations, since all they may need is a different platform edge, instead of expensive lifts, extra ramps, accessible toilets, etc. The works to adapt a platform edge would need to make sure that there isn't too much of a horizontal or vertical gap between the train and the platform edge. The precision involved is surprising to some. I'm afraid I haven't got the figures to hand.
When considering footbridges and other forms of pedestrian track crossings, some less-well-used stations may use road bridges or level crossings to carry pedestrian traffic. Often, these bridges and crossings will have dedicated footpaths that comply with the relevant highway regulations and signage. As such, they are possibly sufficient to also be used as compliant railway crossings without much modification. Your post above may refer to an instance where a "simple" foot crossing was not safe for the number of disabled users anticipated, and there are crossings which are inadequate.
More widely used stations may need extensive and disruptive works, which may not be easy or possible, as has been pointed out. This may be due to the fact that lift shafts would interfere with utilities which are impractical to move, perhaps, or, where they are known for their heritage features, some stations may not be allowed to feature full modifications.
Sometimes works to comply with disability access requirements have to wait until major refurbishment works, when disruption to pedestrian access is likely anyway. Often, this limits economic risks, which must be regulated.