Really we need to see the approach to the sign and how the sign appears and the actual sign itself.
You have valid points on amount of text to read and absorb, but I expect this is a 20mph zone, so that lessens the point as you're going slowly enough to read the sign. The sign is the usual "Flying Motorbike" sign that has been around for donkey's years, but now in widespread use in London. You also mention an advance sign, so can we see that.
Sorry about all this, but unless we can see what you saw, its very difficult to advise on an appeal text. I get the impression you are new to London, because these timed closed streets have been around now for several years.