Looking at the video, I think you have a reasonable case on inadequacy of signage. You say there are two advance signs, but the second one is not placed well at all, and would be taken by most motorists for the road you turned right off, not the one you turned onto. The previous sign is very small and mounted high-up, so could easily be missed or just not noticed.
However, the council are inevitably going to reject your representations, and re-offer you the discount. At that point you can either pay that, or register an appeal at London Tribunals, with the full PCN penalty in play. So a "double-or-quits" gamble.
Because the PCN penalties are so high in London, making the sum payable if lost at LT so large, most people just pay. This is now generating enormous sums of money in London, like around £650 million, (or more), in total, and for me is the scam of the century. Since camera enforcement of moving traffic contraventions came in, councils have been given a Magic Money Tree, and ruthlessly exploit it.
