"The group say the advantage of using planes is that not only are they mobile and there are plenty of them now available, but also that their filtered, one-way airflow systems mirror those of an operating theatre."
Well, the last aircraft with one-way airflow were DC-9 - 30 Anything bigger/youger is using recirculation fans, meaning that used air from the cabin is redistributed back - evenly distributing everything that was exhaled across the cabin. I am not sure about grade of filtering, but definitelly not in micrometer range.
And, the dirtiest thing I saw and worked on was Air Condiiton system of an 10-15 years old airplane from a second class airline. Mixing chambers and ducts layered with worst dirt you can imagine - and best growth bed for germ if you add moisture and right temperature, I suppose.
I think converting hotels, sports halls, etc. would be much easier, methinks.