Seed compost has low nutrient value, as of course will your used MP compost - when push comes to shove, as it appears to have done for you and potentially many others, you could re-use your old compost, giving the mixture a boost with a general purpose fertiliser before use.
Obviously you need to be reasonably confident that you wouldn't be carrying over any pests or diseases from last year.
I'm a little concerned about the situation myself, having virtually no fresh MP compost and still needing quite a few varieties of fresh seeds to sow in about a month (just discovered that despite being an online company, Premier Seeds Direct has suspended operations to protect its staff, which you can't argue with under the present circumstances.)