No, lightships are a completely different thing altogether...
Lizzie only mentions is because Ashford is already a convenient lorry park. No one else mentioned it because they neither live in Ashford, nor particularly care.
Ah, but what travels past Ashford goes on to other linking motorways, like the M25 going over the Thames or under it at Dartford. The other way it clogs up the M25 all the way around to Heathrow / Junction with M4 then Junction with M40. Now if you believe that those motorways can take a very significant rise in lorry and other vehicle traffic, just get onto those roads during the rush hours, or when something goes wrong, which frequently happens.
The Chunnel and the ferries space out the traffic due to their departure / arrival times. With the bridge traffic it would be endless, being only held up with toll like booths checking all documents (after Brexit very strict checking in both directions) and vehicles pulled over for in-depth searches. Now how would the queues for those checks be managed with a far heavier flow of road traffic over the bridge than currently with existing methods of crossing?
As for shipping being a danger to the bridge, it is worth considering that 500-600 ships a day pass through the Dover Narrows, just where Boris is suggesting the bridge should be built. It is already narrow, and a concrete structure with pylons at regular intervals up to the point of the main suspended span would make the shipping lanes very narrow for ships that are up to 400m long, 60m wide, and with a gross tonnage of up to 200,000 tons travelling at a speed of up to 24 knots, heading towards one another. There is then the ferry traffic that crosses these lanes. Not only of course have you got the container ships, but you have numerous huge cruise ships that are overall not much smaller than the former and carry thousands of passengers.
Now a new regime of radar control has greatly reduced the accidents, but they can still happen due to the weather (such as the high wind speeds through the Channel), loss of power, or human failings with such heavy shipping movements. If just one of these shipping monsters were to hit one of the main bridge supports it would cause a disaster for the people on the bridge and the ship.
So the question is, why a bridge anyway? It was decided that a rail tunnel would be built instead of a road tunnel, and we ended up with the Chunnel. Why not now build a road tunnel? Yes, you will still have security, customs issues and road traffic issues, but risk to shipping would be zero, as would the effects of gale force winds that often close our domestic inland suspension bridges, that are far more frequent in the Channel.
But, after all said and done with Brexit who will invest billions in either another crossing of the channel no matter what it is? If they did, I would favour another rail tunnel, but for freight only!