Only male gardeners are participating, in part because of some dubious claims regarding gender-based differences in urine: "There are obvious logistical benefits to limiting it to male members of the team, but also male pee is preferable to women's, as the male stuff is apparently less acidic."
A secondary benefit is anticipated in terms of water conservation:
“An average flush of the lavatory can use anything from four and a half to nine litres of water each time, but what people may not realise is that this water is treated to the same standard as drinking water and shouldn’t be wasted." Urinating outdoors or in the shower is advocated by environmental activists, including Cameron Diaz, as a way of tacking climate change by saving water and energy.
They are tentatively encouraging the public to follow their lead: "“Adding a little pee just helps get it all going; it’s totally safe and a bit of fun too."
Links for the Telegraph and the BBC (where there is an explanatory video).