The Facebook post is very poorly written and overly alarmist in its tone, but I think that its basic message to stay home as much as possible is correct, as is the message not to receive visits from people who do not live in your own household.
It is probably also correct, just based on the arithmetic of spread, that the risk of getting infected from any one encounter today is probably about 2x higher today than the risk from any one encounter 3-4 days ago, since the total number of asymptomatic infectious people in the UK has likely continued to double every 3-4 days, and it is likely to continue growing at that rate until 7-10 days after the start of the more stringent "social distancing measures" announced March 23rd. That means that even if everyone with symptoms stays home, the risk grows between March 23rd and around March 30th / April 2nd.
Depending how strictly we are able to apply the measures as a nation, after that the number of infectious people who don't know they're infectious can start to grow more slowly or even start declining. If people who have symptoms and their households can completely self-isolate as requested, then the number of infectious people who are out and about will be able to start going down quite a lot.
But, if you do need to go out in order to get food, and do not have symptoms yourself, then go out to get food! Just don't go out for longer than needed, stay away from others as much as possible, and do a weekly shop instead of a daily shop if you can in order to reduce the total number of trips. If you have enough food at home to last a few weeks or can get food delivered, then not going out at all is an even better plan. If your household needs to self-isolate due to someone having symptoms, then order a food deliver or call on anyone and everyone you know, or people you don't know, to leave food on your doorstep so that you can avoid going out.
I don't personally know the people who made the video below, but it looks pretty real to me, and it gives the same message as the Facebook post, in less alarmist but still very serious terms. Watch it and judge for yourselves: