I expected better from the BBC.
In your garden, you could mark two metres out with blankets, chalk or masking tape to create little zones for people - no-one from separate households should be sitting next to one another on a bench.
You could ask your guests to set their alarms for every 45 minutes or every hour in a staggered way, then everyone could wash their hands when their alarm goes off. If you're hosting, say to everyone "we're all going to wash our hands once an hour".
You could also have a sign somewhere in the toilet that says "wash your hands".
Even then, remember it's not a risk-free zone. Probably the toilet paper is going to be contaminated - you are definitely taking a risk by doing this.