Self Isolation: What does that mean? Locked in doors? Can you run out doors at all staying close to home and social distancing?
If you can stay active and do some runs I encourage you to stay with your plan and do this race. You need the experience of racing that far. You’ll learn a ton from that even if your preparation is compromised somewhat.
You’ll be that much better prepared the next time to train and race.
If you’ve done Luke’s plan up till mid/late Sept you’ve done the most important part: building a base.
Go for it!