I presume the Blackwall one is that unlit LED sign just at the start of the off-ramp. Then there's another set of height detectors on the same post to catch out anyone who's still not paying attention.

I question who approved that the main lanes ahead of your link have 2.8m/9ft limits but the police warning says vehicles over 4m/13ft will be stopped. Can I take my 10ft truck through or not?

I'm starting to feel a tiny bit of sympathy for drivers that get confused by this.