To answer the first question, I think DfT are pretty close to releasing their notes on the new TSRGD (over 100 pages I have heard). I’m guessing it will either be out on 22 April, or perhaps a few days later.
I’m not an expert on vehicle plating, but I think you can tell whether a lorry is above or below 3.5t by the signs on the back of it. I’m assuming that’s why the London Safer HGV zone is based upon 3.5t. I suggest you discuss this with whoever will be enforcing the restriction, as they probably won’t be happy if it cannot be done visually. (And if no enforcement is lined up, is it worth introducing a restriction that might be widely contravened?)
Does anyone else know more about this?