As far as invasive flora goes round here, christ, the amount of himalayan balsam that grows is insane. I spent quite a while last year whacking down balsam that had started to sprout in and around some of the stands of grass I harvest ergot fungus from.
And its infested a favourite local woodland too, to the extent they hold days for people to go and crack down on it.
Not wrong on the giant hogweed hyke, it really is a nasty bugger if you come into contact with it, burned me once, on my leg, left a black blister about the diameter of a five pence coin that took ages to heal. Didn't really hurt though. Makes me wonder how deep it went.
Its invasive as all hell too, takes years of herbicide intensive treatment to purge it from an area utterly. I've even seen it sprouting up in roadside grass verges. Cut a big plant down last year here because it decided to grow right outside a primary school.