I used recycled gymnasium flooring.  Buy about 50% more than you need to allow for wastage (some broken strips were repairable with wood glue).  I used water based varnish to finish them.  I thought about laying insulation, but decided I would trade a bit of heat loss for extra headroom.
The floor was one of the last things I did - I went for the IVA with a bit of old carpet in place.  I wanted to minimize the amount of work I had to do until I knew I had (or would) pass the IVA test.

Recycycled gymnasium flooring before sanding

Sanded and sorted into sizes.

I used a Clarke Belt sander to remove the top surface

Matching floor and seat leather colour

Use a claw hammer to twist and snap off old nails

There are "male" and "female" ends

Tongue and Grove flooring. Measure out the strips and mix up the joins

I laid wooden strips onto metal floor using gripfill adhesive. I didn't put any other insulation since I wanted to maximise headroom

Floor in place

Resand the floor after laying it