So it's OK for, say, a furloughed office worker to do job related training?
Yes.
Also when I have an HR supremo to hand, is it OK to read or even answer work emails when furloughed in order to be up to date when you return to perform your work?
The odd one here and there wouldn't do any harm. Beyond that, as with most these things, it's a judgement of reasonability. Effectively the Government don't want businesses using furlough to pay wages to staff that are still actively contributing to company performance. You'll never get a ticklist for this kind of thing, and it's not like there's a wealth of caselaw to read.
But for the football, you could easilly justify expecting players to remain fit and train at home. The current guidance is:
While you’re on furlough
Once you are on furlough you will not be able to work for your employer. You can undertake training or volunteer subject to public health guidance, as long as you’re not:
making money for your employer or a company linked or associated to your employer
providing services to your employer or a company linked or associated to your employer
If workers are required to, for example, complete training courses whilst they are furloughed, then they must be paid at least their appropriate minimum wage (NLW/NMW/AMW) for the time spent training, even if this is more than the 80% of their wage that will be subsidised.
Official Source