Youcantchoose - I can see both points of view here - yours and your parents. I have my daughter living very close and she has been fortunate in adding me on to some of her regular deliveries but I hate relying on her and have tried myself to get online with no success. All the promises of extra delivery slots for the 'vulnerable' are fake because what you have to do is divert to the government website to register and that website introduces the word 'extremely' vulnerable and sets out criteria like having had a heart transplant or undergoing chemotherapy - this means that people who have been told they must stay at home like 70+ irrespective of their physical condition, have fallen through the cracks.
So, on three occasions hubby and I have been out in the car to 'look' at Sainsbury's, and yes to give the car a run because we can't let it stand for months, only to find queues around the store's car park perimeter and back onto the main road and of course we have simply come back home, with no provisions. So I do believe we should all stay home and leave only to buy essential provisions, collect medication, or for a short burst of exercise but for those over 70 and unable to get online delivery that's very difficult. You, however, have done what you can for your parents and short of giving them websites of places that will supply them with things like cheese, fruit and veg boxes there is nothing else. Perhaps telling them that if they are sick you can't help, and if they die you won't be able to see them or be at their funeral might shock them into asking a neighbour for a bit more help.