ISTM the way to make distancing possible on public transport is not to keep people off transport, but to increase capacity. If the train runs every 30 seconds, you'll get quite a bit more breathing room than if it runs every five minutes.
[EDIT:] I should have noted, since I've never been to UK this is more of a general observation than a specific recommendation. Certainly most transit systems in USA have the physical capacity, if not the operating budget, to run much more often. Any bus system, however, could buy more buses, if social distancing on transit were a priority. Perhaps I am too suspicious but when I hear "don't ride the train" I translate that to "we're getting rid of the train". Probably not happening in London but you could see it anywhere in USA.