There are commercial starches available in the U.K. Many of them may be the older-fashioned "powdered" starches, though. Here are links to some I found:
http://www.amazon.co.uk/Kershaws-TRA.../dp/B004B3GDPS
http://www.idealmanufacturing.co.uk/...uid%20Starches
The advantage of purchasing a commercial starch, whether liquid or dry, is that they typically include preservatives which help keep the starch from going off.
Cornstarch is basically just powdered corn. You mix it with water. I believe most recipes have you boil the starch at some point, which may help it dissolve and also will help it keep longer. Once mixed, you probably want to keep this in the refrigerator. It will develop mold eventually, but refrigeration slows this process.
Be aware that starches are made from organic materials, and that cornstarch isn't the only thing you can use. You can also use starch made from potatoes, rice, and I'm sure other organic sources.
Think about how you are going to use the starch too. For example, cornstarch and water would work great in a spray bottle, but reconstituted powdered laundry starch would likely be much better for heavily starching yardage (as a substitute for Sta-Flo and water).