I suspect that UK (and WHO) guidelines about face coverings may be about to change shortly.
Here is a simple and practical manual for a homemade mask issued by the Indian Government.
Yes, sure, a homemade mask will not prevent the spread of virus, but they will help limit the spread of virus, in exactly the same way as keeping 2m apart will limit but not prevent the spread of virus.
Just to forestall anyone posting that a home-made mask cannot stop virus spreading and may 'give a false sense of protection blah', we need to remember that the aim of the current Social Distancing guidelines in the UK is to limit the spread in the general population, to slow it down to levels where our Health Service can cope. The aim isn't to prevent it entirely. A mask can help with this.
164.100.117.97/WriteReadData/userfiles/FINAL%20MASK%20MANUAL.pdf