Anything which interferes with your right to use a public right of way such as a barbed wire fence across the path or a heap of manure could be classed as an obstruction. Dense undergrowth is not normally treated as an obstruction but is dealt with under path maintenance. Highway authorities have a duty “to prevent as far as possible the stopping up or obstruction” of paths. You are permitted to remove an obstruction provided that you are a bona fide traveller on the path and have not gone out for the specific purpose of moving the obstruction, and that you remove only as much as is necessary to get through. If you can easily go round the obstruction without causing any damage, then you should do so.