Dnr orders are obviously appropriate in some cases. You don't want to wake up in a state worse than death - with catastrophic brain damage, for instance. However, it is a frightening prospect and should surely be done on an individual basis. According to the local paper The Argus, care homes in Brighton and Hove say they have been asked to get residents to sign "in industrial quantities." In other places, care homes report that they feel pushed into pressurising residents and families to sign. This sounds less like individual circumstances, and more like getting a lot of very old bed-blockers out of our hospitals.
I was interested to see that Harry's deceased brother had Down Syndrome. Mencap are unhappy about the suggestion that people with learning difficulties might be "too frail" to be ventilated and would, presumably, be excluded. They point out that many people with learning difficulties are physically robust.
In the USA, the national advice is that all lives are of equal value but some states - Alabama, for instance - included people with "severe mental retardation" on the list of those not to ventilate. I don't know if this has been put into practice or whether the relevant charities have been able to protest.
Personally, I can't imagine many UK doctors deciding that a person could not live based on his/her intelligence, but I bet Dominic Cummings would....