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I very much doubt they'll use that section by the wall for a London League round, as it's too narrow (even by Herne Hill standards).
As for being an obstacle, cyclocross is a very inclusive sport and everyone knows there will be slower riders that the faster people will pass, so everyone passing should warn you in advance which side they'll pass on.
If that hasn't convinced you then the London X League rounds are here;
http://www.londonxleague.co.uk/calander-2014-2015/
I believe the 5th October round is at Hog Hill.
It was the first place I did CX and I found the bit over by the wall in particular to be too scary. As in, I was at the top of the drop, staring down it, and couldn't force myself to do it even though I was totally embarrassed to be in tears, scared to ride my bike down a hill, in front of a whole load of kids.
Having ridden a few times since then I think I could probably make my way around the course on my own but not with other people around. Cyclocross courses have obstacles built into them already without me being the mobile (or not so mobile) chicane.
The problem is 100% in my head.