You can only claim after travel. You may therefore want to investigate other routes that keep you in First for most of the flight, usually YouFirst is amenable to sensible suggestions in this space.

The amount you can claim is pro-rated by distance and you have to be careful about exact taxes and fees, as opposed to airline surcharges. The calculation is known as the Mennens Formula and if you look in the wiki of the EC261 thread you can see the way to calculate this. As also referenced in that thread, BA have an Involuntary Refund formula, which will depend on your fare basis. Sometimes this is more generous than EC261, often it's quite similar (to the extent it's not worth complaining about) and sometimes it is significantly less generous. Given the low fare here, I suspect it was a special fare or a J upgrade offer, in which case BA's Involuntary Fare calculation is not going to be generous. You'll need some patience in this space, but what you can do now is do an approximate calculation on Mennens to see what a reasonable outcome look like. The EC261 calculation can't be massive either, given the numbers provided, hence it may be better to restructure the booking.

This is not affected by the Covid pandemic, there is no exception circumstance on this one.