The definition of disability is set by the Equality Act 2010. I guess other countries have a different and much narrower definition. Basically ours says that it’s not the medical condition that qualifies someone as disabled but that the effect on everyday life is the consideration. There are a couple of conditions such as MS and cancer which automatically meet this definition but wouldn’t automatically qualify for disability benefits unless the condition was terminal within the next twelve months.
I do think a poster upthread identified a problem that needs sorting out, in that the benefit cap doesn’t apply to any household where someone is in receipt of disability benefits. That, among other things is a disincentive to work, despite the fact that disability benefits don’t assess the ability to work of the person claiming them. They’re purely to support the extra cost of living with a disability.
ETA. I also think another problem with the definition of disability is that it includes health conditions which are short term and from which people can and do recover. At one time disability benefits were just that - meant for those who had life long disability from which they would not recover.
The PIP system has a built in safeguard in the length of benefit award being linked to how likely the condition is to improve in the short term, but in the interim the onus is still on the claimant to report any change which would affect their entitlement - which doesn’t happen for obvious reasons. To my mind better savings would be achievable if the minimum duration of a condition (currently 12 months) was increased - this would filter out the more minor conditions which fly under the radar of self reported recovery.
Better designed assessments using properly qualified medics would be a money saver too. It result in better and fairer benefit decisions, and there wouldn’t be the need for hugely expensive appeals in order to get a fair decision.