Yes wild boar are present along the Kent/Sussex border (and also in the West I believe?). There was a good article on their field-signs in British Wildlife magazine - so trackers take note!
As for invasive plants, we have problems with japanese knotweed, himalayan balsam and giant hogweed around here. But some of the worst offenders are the non-native aquatics such as floating pennywort and parrots feather. They can spread very quickly. They are still being sold from garden and aquatic centres.
Unfortunately, often the only way to control these plants effectively is to use herbicide, but if using it near water you must get approval from the Environment Agency. Only certain herbicides are approved. With some invasive plants you should be very careful not to go strimming/cutting (without carefull controls) as the plants can regenerate from very small fragments and make the problem worse (this is particularly true with japanese knotweed). By the way, Japanese Knotweed is edible!
Also be aware that Giant Hogweed can cause severe blisters if the sap comes into contact with skin. (a number of other plants can also have this effect).