Rain water leaching doesn’t concern me in my veg growing personally. With a thick mulch of compost I am happy that my winter beds are sufficiently ready for spring planting and sowing as is. Plastic sheeting can’t hurt…other than encourage slugs in our damp climate. This relates to green manures too as the slimey critters enjoy vegetation to live in – another potential reason for not bothering with them on a small scale allotment etc.
Green manures can be very beneficial though depending on your circumstances and different green manures bring different benefits. Elliot Colman is convinced that the roots of grass do something very beneficial to soil, especially for a following veg crop. He grows on a larger scale though than most of us and often combines green manures with veg crops – but in a drier climate.
Chap called Pete on here from years ago grows a lot of green manures very successfully and Iain Tolhurst does this too, with his vegan veg ethos.
There’s a lot of variables….it’s up to you and possibly your time constraints. Good luck with the coming season.