being part Yorkshire and not liking to spend more money than I need (!), two years ago I found an ideal way to grow sweet peas with a deep rooting system. I cut a 1-litre milk carton roughly in half. Then I invert the top half and stand it in the bottom part (to contain the compost). The two parts together thus form a funnel-shape pot, quite deep but not so wide that you have to use a lot of compost! I sow one sweet pea per pot (usually soaked on damp tissue for a day or two first till it splits). Assuming your local Council recycles plastic, the cartons can be rinsed out when finished with and sent for recycling in the usual way.
On the subject of milk cartons - you do buy British milk, do you, to support our struggling farmers - we can't expect them to be conscientious custodians of teh countryside if they're nearly skint!
Hope it's not too late for at least some sweet pea lovers out there..
I make drainage holes in the bottom part and stand it on the usual gravel tray to contain drips and cut diagonally along the top part to allow the light to get to the compost freely.