I could see the season being cancelled if this strain hits the rest of the UK and hangs around, especially if the vaccine isn’t effective against it and so there is no date for a scheduled restart. Timings are going to be tight with Euro 2020 anyway so I could see them saying cancel this season and start again in August. I doubt at present they’d want to extend this season again so affecting 2020 and possibly quite badly the start of next season.
I seriously doubt they would relegate anyone yet, it is still to early even though we’d be the favourites obviously but the biggest issue would be the other leagues with promotion or relegation. No team would accept going down if they are a point or two behind safety or other teams being promoted if they are ahead by a few points.