hogweed
My daytime job is treating many of the alien invasive plants you have mentioned here.
Himalayan balsam can be easily controlled by manually removing the plants before they seed, because they are annuals the root stock dies every year and the plants spread by the explosive seed pods. Cut out the annual injection of new seeds and in a few years you should have eliminated the plant but you have to be meticulous with its removal. This way there is no collateral damage done by herbicide application. Unfortunately it is probably to late to consider it possible to eradicate this from our countryside.
Giant hogweed is a fish of a different kettle. From day one you have to assume you may have 10 years of treatment and monitoring ahead of you, because each plant can produce 10,000 seeds and these can be viable for up to 10 years. This means, on an established site, that even if you manage to eradicate all the plants before they seed, you may still have up to 10 years worth of viable seeds already in the ground. I wouldn`t reccommend cutting this plant because of its dangerous sap, which is a chemical that makes your skin extremely photosensitive, this will bring you out in huge blisters on any exposure to sunlight. Spray with a reputable glyphosate herbicide before the flower heads develop. Re-treat throughout the summer because as the large plants die off other seeds will germinate to take their place, these must also be treated before seed heads develop. If you are using herbicide adjacent to the watercourse you will need EA permission and hold a current PA1 6AW herbicide permit. Attention to detail is the key to effectively treating this plant.
Once we have these under control, we only have to concern ourselves with; japanese knotweed (3 species; fallopia japonica, fallopia sachelinensis and fallopia x bohemica as well as a few hybrids which we have little clue as to how vigorous they might be) floating pennywort, azolla, australian swamp stone crop, creeping water primrose and parrots feather just to keep us busy.
Mic