Wild Plants as a Food Source

[T-oddity]

It is most likely that these plants are already plenty full in your local environment, you don't realy need to be buying seed.
I you have any water courses or wetlands near you, reed mace is a really good source of carbs all year round and very easy to introduce.

You could use guerrilla gardening seed bomb technique to spread any kind of seed that you wish.

http://www.guerrillagardening.org/ggseedbombs.html.

Something that I have been doing for awhile on similar lines is to raise fruit trees and bushes from seed and cuttings to plant out in any rough undisturbed area's that I can find, but to be honest not all survive, expect at least 30+% losses.
The real difficulty though is finding places to grow stuff that is truly undisturbed, nearly all of our hedgerow and verges are managed to some extent, even those that look like they aren't are done on a 2 or 3 year cycle, such as some river banks, motorway embankments.

A word of warning about Burdock seed, if you choose to collect your own seed from the wild, be aware that the seeds are covered in very fine hairs that can get stuck in the pores of your skin, (itching powder on steroids) I nearly blinded myself collecting these seeds by trying to separate the seeds from the hairs by blowing on them, they promptly blew back into my face, they don't brush or wash off, I think I ended up using sticky tape to lift the hairs out of my skin.