Right, while the via Denmark Hill and Clapham Junction is acceptable to both Not Underground (£28.90) and Not Via London (£29.80), the reason the former isn't offered is because of this easement, which bans Clapham Junction if you're going from Elephant to Brighton (amoungst other locations) specifically for Not Underground routes. Obviously not particularly sensible, given Not Via London is allowed, but easements often have strange consequences.